Brain usually slows down when the activity of a person's central nervous system is reduced. The correct option is A.
What is central nervous system?
The brain and spinal cord make up the central nervous system. There are three major functions are to receive sensory data, processing information, and submit out motor signals.
It receives sensory information from the nervous system and directs the responses of the body. Without its activity, brain will slow down.
Thus, the correct option is A.
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