对于钢轨焊后正火处理时,待钢轨焊接头低于()℃时,方可重新加热。
A.400
B.200
C.500
D.600
参考答案:C
I don't have a partner _______. [ ]
A、to practice English
B、practice English
C、practicing English with
D、to practice English with
正常视网膜所以能保持干燥而透明,是因其有________和________两种屏障。