Identify the topic sentence in the given paragraph. Humans need sleep to help their bralns function properly. The brain needs a minimum number of hours to rest and organize itself. The number of hours depends on age. For example, babies need an average of 14 to 16 hours of rest daily, while adults can function on about 6 to 8 hours. Scientists are not completely sure what the brain does during sleep periods, but they do know that the brain sorts and stores information and replaces chemicals when a person sleeps.

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Comedians like to joke that the reason we haven’t been visited by intelligent life from elsewhere in the universe is that aliens have been monitoring earth’s broadcasts of intellectually embarrassing tv programs, like gilligan’s island, fear factor, the jersey shore, and the jerry springer showand so consider us far too primitive to merit a visit. let’s check the assertion that aliens could have been receiving them. tv programs are broadcast at a frequency of about 100mhz with about 100kw of total power in 30frames per second, which emanatesroughly uniformly in all directions. assume that interstellar space transmits these broadcasts without attenuation. no matter how smart they are, aliens would require at least one photon per frame to interpret our signals. findthe number of photons per unit time per unit area reaching a receiver on a possible planet in the nearest star system, which is about 4 light-years away (a light-year is the distance light travels in a year). if aliens aimed a receiver or detector directly at earth, how big (in diameter) would it have to be to receive a photon per frame