Please answer the following question: Information: - A lustrum (plural lustra) was a term for a five-year period in Ancient Rome. - The modern Olympic Games or Olympics are leading international sporting events featuring summer and winter sports competitions in which thousands of athletes from around the world participate in a variety of competitions. The Olympic Games are considered the world's foremost sports competition with more than 200 nations participating. The Olympic Games are held every four years, with the Summer and Winter Games alternating by occurring every four years but two years apart. - Latin (Latin: ) is a classical language belonging to the Italic branch of the Indo-European languages. The Latin alphabet is derived from the Etruscan and Greek alphabets. - A decade is a period of 10 years . The word is derived ( via French and Latin ) from the Ancient Greek dekas which means ten . Other words for spans of years also come from Latin : biennium , triennium , quadrennium ( 4 years ) , lustrum ( 5 years ) , century ( 100 years ) , millennium ( 1000 years ) . - A leap year (also known as an intercalary year or a bissextile year) is a calendar year containing one additional day (or, in the case of lunisolar calendars, a month) added to keep the calendar year synchronized with the astronomical or seasonal year. Because seasons and astronomical events do not repeat in a whole number of days, calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track. By inserting (also called "intercalating") an additional day or month into the year, the drift can be corrected. A year that is not a leap year is called a common year. - English is a West Germanic language that was first spoken in early medieval England and is now a global "lingua franca". Named after the Angles, one of the Germanic tribes that migrated to England, it ultimately derives its name from the Anglia (Angeln) peninsula in the Baltic Sea. It is most closely related to the Frisian languages, although its vocabulary has been significantly influenced by other Germanic languages in the early medieval period, and later by Romance languages, particularly French. English is either the official language or one of the official languages in almost 60 sovereign states. It is the most commonly spoken language in the United Kingdom, the United States, Canada, Australia, Ireland, and New Zealand, and is widely spoken in some areas of the Caribbean, Africa, and South Asia. It is the third most common native language in the world, after Mandarin and Spanish. It is the most widely learned second language and an official language of the United Nations, of the European Union, and of many other world and regional international organisations. It is the most widely spoken Germanic language, accounting for at least 70% of speakers of this Indo-European branch. - A millennium (plural millennia or millenniums) is a period equal to 1000 years, also called kiloyears. It derives from the Latin ', thousand, and ', year. It is often, but not always, related to a particular dating system. - A century (from the Latin "centum", meaning "one hundred"; abbreviated c.) is 100 years. Centuries are numbered ordinally in the English language along with many others (e.g. "the 7th century AD/CE"). A centenary is a hundredth anniversary or a celebration of this, typically the remembrance of an event which took place a hundred years earlier. Its adjectival form is centennial. - A year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by changes in weather, the hours of daylight, and, consequently, vegetation and soil fertility. In temperate and subpolar regions around the globe, four seasons are generally recognized: "spring", "summer", "autumn" and "winter". In tropical and subtropical regions several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual "wet" and "dry" seasons are recognized and tracked. - A quadrennium (pl. quadrennia or quadrenniums) is a period of four years, most commonly used in reference to the four-year period between each Olympic Games. It is also used in reference to the four-year interval between leap years, for example when wishing friends and family a "happy quadrennium" on February 29. - A day is a unit of time. In common usage, it is either an interval equal to 24 hours or daytime, the consecutive period of time during which the Sun is above the horizon. The period of time during which the Earth completes one rotation with respect to the Sun is called a "solar day". Several definitions of this universal human concept are used according to context, need and convenience. In 1960, the second was redefined in terms of the orbital motion of the Earth, and was designated the SI base unit of time. The unit of measurement "day", redefined in 1960 as 86 400 SI seconds and symbolized "d", is not an SI unit, but is accepted for use with SI. A civil day is usually 86 400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time. The word "day" may also refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns of humans and many other species are related to Earth's solar day and the day-night cycle (see circadian rhythms). - Ancient Rome was an Italic civilization that began on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to become one of the largest empires in the ancient world with an estimated 50 to 90million inhabitants (roughly 20% of the world's population) and covering 5.0millionsquarekilometers at its height in AD 117. 'decade' is related to which object entity through the relation of 'subclass of'? Choices: - alphabet - anniversary - calendar - calendar year - case - city - civilization - competition - course - day - england - english - family - french - greek - height - interval - keep - language - latin - official - olympic games - peninsula - plural - reference - romance languages - second - spanish - spring - term - time - unit of time - variety - vocabulary - world - year
A:
unit of time