Muscles in animals are often found in pairs, because
a. muscles can only pull.
b. muscles need to be able to both push and pull.
c. muscles can only push.
d. muscles can only produce a force as they relax.

The correct answer is muscles can only pull.

The movement of arm takes place by the relaxation and contraction of muscles. Muscles in animals are in pair because muscles can only pull, when a pair of muscle is present in animals one muscle relaxes and the other one contracts. An example of this is biceps and triceps of arm. The triceps  of the arm relaxes while the biceps of the arm  contracts, this movement helps to lift the arm. The reverse also takes place in the arm.

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