Muscles in animals are often found in pairs, because

A. muscles need to be able to both push and pull.

B. muscles can only push.

C. muscles can only produce a force as they relax.

D. muscles can only pull.

Muscles are able to produce force only while they are contracting. For example, the human bicep produces force only when it is contracted to pull the forearm upwards. In order to straighten the arm once again, a pushing force is needed which is provided by the tricep. Therefore, because muscles need to be able to both push and pull, they work in pairs.