This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.

Q: Passage: Uranium is a naturally occurring element that can be found in low levels within all rock, soil, and water. Uranium is the 51st element in order of abundance in the Earth's crust. Uranium is also the highest-numbered element to be found naturally in significant quantities on Earth and is almost always found combined with other elements. Along with all elements having atomic weights higher than that of iron, it is only naturally formed in supernovae. The decay of uranium, thorium, and potassium-40 in the Earth's mantle is thought to be the main source of heat that keeps the outer core liquid and drives mantle convection, which in turn drives plate tectonics. Question: Where does uranium rank among elements in terms of its abundance in the Earth's crust?

A: 51st
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Q: Passage: With the Conference of Berlin of 1884, Portuguese Africa territories had their borders formally established on request of Portugal in order to protect the centuries-long Portuguese interests in the continent from rivalries enticed by the Scramble for Africa. Portuguese Africa's cities and towns like Nova Lisboa, Sá da Bandeira, Silva Porto, Malanje, Tete, Vila Junqueiro, Vila Pery and Vila Cabral were founded or redeveloped inland during this period and beyond. New coastal towns like Beira, Moçâmedes, Lobito, João Belo, Nacala and Porto Amélia were also founded. Even before the turn of the 20th century, railway tracks as the Benguela railway in Angola, and the Beira railway in Mozambique, started to be built to link coastal areas and selected inland regions. Question: During what period were the Portugese Africa's cities founded or redeveloped?

A: Scramble for Africa
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Q: Passage: A variety of games for the FDS were released by Nintendo (including some like Super Mario Bros. which had already been released on cartridge) and third party companies such as Konami and Taito. A few unlicensed titles were made as well. However, its limitations became quickly apparent as larger ROM chips were introduced, allowing cartridges with greater than 128k of space. More advanced memory management chips (MMC) soon appeared and the FDS quickly became obsolete. Nintendo also charged developers considerable amounts of money to produce FDS games, and many refused to develop for it, instead continuing to make cartridge titles. Many FDS disks have no dust covers (except in some unlicensed and bootleg variants) and are easily prone to getting dirt on the media. In addition, the drive use a belt which breaks frequently and requires invasive replacement. After only two years, the FDS was discontinued, although vending booths remained in place until 1993 and Nintendo continued to service drives, and to rewrite and offer replacement disks until 2003. Question: Many FDS disks lack what?

A:
dust covers
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