A Normal, Healthy Animal Never Contracts the Flexor Muscles and the Extensor

Multiple Choice

 A normal, healthy animal never contracts the flexor muscles and the extensor muscles of the same leg at the same time. Why not?

A) When the interneuron sends excitatory messages to one, inhibitory messages go to the other.
B) Both muscles are mechanically connected in a way that makes it impossible for both to contract at the same time.
C) Such coordination is learned through prenatal movement.
D) Both muscles are controlled by branches of the same axon.