单相全波整流X线机，高压整流器的个数是（）
A.6
B.4
C.2
D.8
E.12
参考答案：B
解析：单项全波桥式整流只能采用四管整流。
下列影响我国经济社会可持续发展的首要问题是[ ]
A．资源问题
B．人口问题
C．环境问题
D．社会问题
In history, many famous women _____ bravely_____ equal rights. [ ]
A. fought; for
B. had fought; for
C. fought; with
D. have fought; against
