—Do you have anything to say for yourselves?

—Yes，there is one point________ we must insist on.

A．why

B．where

C．how

D．/

N型半导体和P型半导体所呈现的电性分别为：（）

A.正电，负电

B.负电，正电

C.负电，负电

D.中性，中性