—Mr. Xu, can you tell us how to learn English well?

—Sure. But remember nothing can be learned        hard work.

A．by

B．with

C．without

D．for

在抽样调查中，抽样误差的大小关系为（）

A.整群抽样>系统抽样>单纯抽样>分层抽样

B.整群抽样>单纯抽样>系统抽样>分层抽样

C.单纯抽样>整群抽样>系统抽样>分层抽样

D.分层抽样>系统抽样>单纯抽样>整群抽样

E.系统抽样>单纯抽样>整群抽样>分层抽样