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 answer is letter B.

Muscles in animals are often or almost always found in pairs because they need to be able to both push and pull. Pushing and pulling are the basic movements that the skeletal system can perform. It occurs when individuals are lifting, walking, running, among others. Almost all activities need the push and pull motion of muscles to balance out movements. Without this mechanism, we might easily fall over while walking (e.g. hamstrings are pulling back while the quadriceps are pushing forward).