For the molecular weight distribution, which one of the followings is not correct?

For the molecular weight distribution, which one of the followings is not correct?
A.There is a considerable variation in molecular weight of polymers
B.The molecular weight distribution can be expressed by the ratio Mw/Mn.　
C.A polymer sample can be considered as a mixture of chains having different lengths　
D.The molecular weights of polymers are usually uniform
正确答案： The molecular weights of polymers are usually uniform