When a muscle contracts, it performs work. it does this by a) converting chemical energy to mechanical energy. b) converting mechanical energy into heat energy. c) converting mass into energy. d) creating energy.

The answer is A.
The brain sends signals by chemical reactions that occur in the neurons to the muscle cells.
The muscle cells, after receiving the signals sent by brain, contract and expand necessarily to execute the action, in other words, it performs work.
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The relationship of both of them