Question: What type of vegetation is in the central part of the country called the 'Giant of Africa'?

full_context: Document 1: Chad: Chad has several regions: a desert zone in the north, an arid Sahelian belt in the centre and a more fertile Sudanian Savanna zone in the south. Lake Chad, after which the country is named, is the largest wetland in Chad and the second - largest in Africa. The capital N'Djamena is the largest city.
Document 2: Nigeria: According to a 2001 report from The World Factbook by CIA, about 50% of Nigeria's population is Muslim, 40% are Christians and 10% adhere to local religions. But in some recent report, the Christian population is now sightly larger than the Muslim population. An 18 December 2012 report on religion and public life by the Pew Research Center stated that in 2010, 49.3 percent of Nigeria's population was Christian, 48.8 percent was Muslim, and 1.9 percent were followers of indigenous and other religions, or unaffiliated. Additionally, the 2010s census of Association of Religion Data Archives has reported that 46.5 percent of the total population is Christian, slightly bigger than the Muslim population of 45.5 percent, and that 7.7 percent are members of other religious groups.
Document 3: Geography of Nigeria: Nigeria, like the rest of West Africa and other tropical lands, has only two seasons. These are the dry season and the rainy season. The dry season is accompanied by a dust laden airmass from the Sahara Desert, locally known as Harmattan, or by its main name, The Tropical Continental (CT) airmass, while the rainy season is heavily influenced by an airmass originating from the south atlantic ocean, locally known as the south west wind, or by its main name, The Tropical Maritime (MT) airmass. These two major wind systems in Nigeria are known as the trade winds.
Document 4: Nigeria: Nigeria is a religiously diverse society, with Islam and Christianity being the most widely professed religions. Nigerians are nearly equally divided into Christians and Muslims, with a tiny minority of adherents of Animism and other religions. According to one recent estimate, over 40% of Nigeria's population adheres to Islam (mainly Sunni, other branches are also present). Christianity is practised by 58% of the population (among them 74% are Protestant, 25% Roman Catholic, 1% other Christian). Adherents of Animism and other religions collectively represent 1.4% of the population.
Document 5: Nigeria: Among Christians, the Pew Research survey found that 74% were Protestant, 25% were Catholic, and 1% belonged to other Christian denominations, including a small Orthodox Christian community. In terms of Nigeria's major ethnic groups, the Hausa ethnic group (predominant in the north) was found to be 95% Muslim and 5% Christian, the Yoruba tribe (predominant in the west) was 55% Muslim, 35% Christian and 10% adherents of other religions, while the Igbos (predominant in the east) and the Ijaw (south) were 98% Christian, with 2% practising traditional religions. The middle belt of Nigeria contains the largest number of minority ethnic groups in Nigeria, who were found to be mostly Christians and members of traditional religions, with a small proportion of Muslims.
Document 6: Namibia: Namibia is the driest country in sub-Saharan Africa and depends largely on groundwater. With an average rainfall of about 350 mm per annum, the highest rainfall occurs in the Caprivi in the northeast (about 600 mm per annum) and decreases in a westerly and southwesterly direction to as little as 50 mm and less per annum at the coast. The only perennial rivers are found on the national borders with South Africa, Angola, Zambia, and the short border with Botswana in the Caprivi. In the interior of the country surface water is available only in the summer months when rivers are in flood after exceptional rainfalls. Otherwise, surface water is restricted to a few large storage dams retaining and damming up these seasonal floods and their runoff. Thus, where people don’t live near perennial rivers or make use of the storage dams, they are dependent on groundwater. The advantage of using groundwater sources is that even isolated communities and those economic activities located far from good surface water sources such as mining, agriculture, and tourism can be supplied from groundwater over nearly 80% of the country.
Document 7: Sahara: The desert comprises much of North Africa, excluding the fertile region on the Mediterranean Sea coast, the Atlas Mountains of the Maghreb, and the Nile Valley in Egypt and Sudan. It stretches from the Red Sea in the east and the Mediterranean in the north to the Atlantic Ocean in the west, where the landscape gradually changes from desert to coastal plains. To the south, it is bounded by the Sahel, a belt of semi-arid tropical savanna around the Niger River valley and the Sudan Region of Sub-Saharan Africa.
Document 8: Geography of Nigeria: With the Intertropical Convergence Zone (ITCZ) swinging northward over West Africa from the Southern Hemisphere in April, heavy showers coming from pre-monsoonal convective clouds mainly in the form of squall lines also known as the north easterlies formed mainly as a result of the interactions of the two dominant airmasses in Nigeria known as the Maritime tropical (south westerlies) and the Continental tropical (north easterlies), begins in central Nigeria while the Monsoons from the south atlantic ocean arrives in central Nigeria in July bringing with it high humidity, heavy cloud cover and heavy rainfall which can be daily occurrence lasting till September when the monsoons gradually begin retreating southward to the southern part of Nigeria. Rainfall totals in central Nigeria varies from 1,100 mm (43.3 in) in the lowlands of the river Niger Benue trough to over 2,000 mm (78.7 in) along the south western escarpment of the Jos Plateau.
Document 9: Nigeria: Nigeria is often referred to as the "Giant of Africa", owing to its large population and economy. With approximately 182 million inhabitants, Nigeria is the most populous country in Africa and the seventh most populous country in the world. Nigeria has one of the largest populations of youth in the world. The country is viewed as a multinational state, as it is inhabited by over 500 ethnic groups, of which the three largest are the Hausa, Igbo and Yoruba; these ethnic groups speak over 500 different languages, and are identified with wide variety of cultures. The official language is English. Nigeria is divided roughly in half between Christians, who live mostly in the southern part of the country, and Muslims in the northern part. A minority of the population practise religions indigenous to Nigeria, such as those native to Igbo and Yoruba peoples.
Document 10: Sahara: To the north, the Sahara skirts the Mediterranean Sea in Egypt and portions of Libya, but in Cyrenaica and the Maghreb, the Sahara borders the Mediterranean forest, woodland, and scrub ecoregions of northern Africa, all of which have a Mediterranean climate characterized by hot summers and cool and rainy winters. According to the botanical criteria of Frank White and geographer Robert Capot-Rey, the northern limit of the Sahara corresponds to the northern limit of date palm cultivation and the southern limit of the range of esparto, a grass typical of the Mediterranean climate portion of the Maghreb and Iberia. The northern limit also corresponds to the 100 mm (3.9 in) isohyet of annual precipitation.
Document 11: Nigeria: Everything in between the far south and the far north is savannah (insignificant tree cover, with grasses and flowers located between trees). Rainfall is more limited, to between 500 and 1,500 millimetres (20 and 60 in) per year. The savannah zone's three categories are Guinean forest-savanna mosaic, Sudan savannah, and Sahel savannah. Guinean forest-savanna mosaic is plains of tall grass interrupted by trees. Sudan savannah is similar but with shorter grasses and shorter trees. Sahel savannah consists of patches of grass and sand, found in the northeast. In the Sahel region, rain is less than 500 millimetres (20 in) per year and the Sahara Desert is encroaching. In the dry north-east corner of the country lies Lake Chad, which Nigeria shares with Niger, Chad and Cameroon.
Document 12: Sahara: To the south, the Sahara is bounded by the Sahel, a belt of dry tropical savanna with a summer rainy season that extends across Africa from east to west. The southern limit of the Sahara is indicated botanically by the southern limit of Cornulaca monacantha (a drought-tolerant member of the Chenopodiaceae), or northern limit of Cenchrus biflorus, a grass typical of the Sahel. According to climatic criteria, the southern limit of the Sahara corresponds to the 150 mm (5.9 in) isohyet of annual precipitation (this is a long-term average, since precipitation varies annually).
Document 13: Yoruba people: The Yoruba people (Yoruba: Ìran Yorùbá, lit. 'Yoruba lineage'; also known as Àwon omo Yorùbá, lit. 'Children of Yoruba', or simply as the Yoruba) are an ethnic group of southwestern and north - central Nigeria, as well as southern and central Benin. Together, these regions are known as Yorubaland. The Yoruba constitute over 40 million people in total. The majority of this population is from Nigeria, and the Yoruba make up 21% of the country's population, according to the CIA World Factbook, making them one of the largest ethnic groups in Africa. The majority of the Yoruba speak the Yoruba language, which is tonal, and is the Niger - Congo language with the largest number of native speakers.
Document 14: Nigeria: With the majority of Nigeria's populace in the rural areas, the major languages of communication in the country remain indigenous languages. Some of the largest of these, notably Yoruba and Igbo, have derived standardised languages from a number of different dialects and are widely spoken by those ethnic groups. Nigerian Pidgin English, often known simply as 'Pidgin' or 'Broken' (Broken English), is also a popular lingua franca, though with varying regional influences on dialect and slang. The pidgin English or Nigerian English is widely spoken within the Niger Delta Regions, predominately in Warri, Sapele, Port Harcourt, Agenebode, Ewu, and Benin City.
Document 15: Nigeria: Nigeria is often referred to as the ``Giant of Africa '', owing to its large population and economy. With 186 million inhabitants, Nigeria is the most populous country in Africa and the seventh most populous country in the world. Nigeria has the third - largest youth population in the world, after India and China, with more than 90 million of its population under age 18. The country is viewed as a multinational state as it is inhabited by over 500 ethnic groups, of which the three largest are the Hausa, Igbo and Yoruba; these ethnic groups speak over 500 different languages and are identified with a wide variety of cultures. The official language is English. Nigeria is divided roughly in half between Christians, who live mostly in the southern part of the country, and Muslims, who live mostly in the north. A minority of the population practise religions indigenous to Nigeria, such as those native to the Igbo and Yoruba ethnicities.
ground_truth: savannah
sub_queries: Which country is called the 'Giant of Africa'? Answer: Nigeria
What type of vegetation is in central Nigeria ? Answer: savannah