Answer:
B. While one muscle in the pair contracts, the other relaxes
Explanation:
If we consider by elimination:
A -- If both muscles contract at the same time, nothing would happen except energy being depleted. Think of when you hold your arm out straight and try to flex those muscles. The arm doesn't move.
C -- Both muscles extend at the same time, this is similar to A.
D -- If both muscles do nothing, then nothing happens!
Thus B is the answer, since every force requires an opposing force, so when one muscle contracts, the other relaxes to allow the movement to take place.
