What Is the Most Important Difference Between Australopithecus Afarensis and the Modern

What is the most important difference between Australopithecus afarensis and the modern apes?

A) Au. afarensis had a narrow chest, whereas living apes have a barrel chest.
B) Au. afarensis had better color vision than apes.
C) Au. afarensis had lost its prehensile tail.
D) Au. afarensis had increased cranial capacity.
E) Au. afarensis was bipedal.