直流输电的功率反送也称为（）
A.潮流反转
B.逆变
C.潮流反向
D.整流
参考答案：A, C
— How many students are there in the room?
— Twenty _____, I'm not sure. [ ]
A. or so
B. or rather
C. about
D. more than
树木无性繁殖为什么要营建采穗圃？
