Which Statement About Joints Is True

Which statement about joints is true?

A) At joints, muscles work in agonistic pairs.
B) Movement occurs at a joint when one muscle of an antagonistic pair contracts and the other member of the pair relaxes.
C) Flexors are muscles that straighten a joint.
D) If both muscles of an antagonistic pair contract, then the joint moves.
E) When the extensor muscles that operate your knee joint contract, your leg bends at the knee.