一般通过改变（）来改变单结晶体管触发电路的震荡频率。
A、Ｒe
B、C
C、L
D、U
参考答案：B
—English is______ too difficult for me. I can't learn it well.
—Don't give up. Nothing is difficult if you work hard. [ ]
A. seldom
B. usually
C. never
D. always
目前我国男性发病率最高的恶性肿瘤是（）
A.胃癌
B.肺癌
C.肝癌
D.结直肠癌
E.食管癌
