A muscle cell contracts or expands when

Walking

Explanation:

A muscle cell usually contracts or expands simultaneously when one is working or doing some activities like running, exercises, house chores e.t.c

Muscles are flexible fibers that are very essential appendages for locomotion in human beings.

For muscle cells to contract, they are pulled together thereby causing tension in the muscle. When they are relaxed, they move away from one another.

1. desertification

2. captive breeding

3. black rhinoceros

4. renewable resources

explanation:

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