Mr. Li is a famous writer , he           already           four books . And they all sell well now. [ ]

A. has, written  

B. have , written    

C. has , write  

D. has wrote

在晶闸管整流主电路中，接入快速空气开关，对晶闸管起（）保护作用。

A.过流；

B.过压；

C.过流、过压；

D.过载。