Make a tentative guess _____ Peking Man _____ have done and used thousands of years ago.

A．that; may

B．that; must

C．what; may

D．what; should

自顶向下开发方法的优点之一是( )。

A．开发周期短

B．开发费用低

C．对系统分析人员要求低

D．能支持信息系统整体化