目前生产上推广的玉米杂交种以（）为主。
A.顶交种
B.双交种
C.单交种
D.三交种
参考答案：C
After graduation from college, he began to wander from city to city, ________a suitable job. [ ]
A. hunting for
B. taking on
C. looking after
D. bringing up
固定连接的双母线差动保护，当母线固定连接方式破坏后，母线保护应投在位置。
A、有选择
B、无选择
C、停用
