_______ is even important is ______ as the earth cooled down, water began to appear on its surface.

A．That, that

B．What, that

C．What, what

D．It, that

（2007）晶体管单管放大电路如图a）所示时，其中电阻RB可调，当输入Ui、输出U0的波形如图b）所示，输出波形：（）

A.出现了饱和失真，应调大RB

B.出现了饱和失真，应调小RB

C.出现了截止失真，应调大RB

D.出现了截止失真，应调小RB