Read the paragraph and choose the sentence that best fills the gap: 'Scientists have long studied the effects of sleep deprivation. ___. Without adequate rest, cognitive functions such as memory and attention decline rapidly.' Which sentence fits best?
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A
Sleep is a biological necessity for all mammals.
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B
Recent research confirms that adults need seven to nine hours per night.
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C
Dreams occur primarily during REM sleep cycles.
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D
Insomnia is classified as a sleep disorder by the WHO.