轴承润滑是为了（）。
A、减小摩擦
B、冷却轴承
C、吸振
D、防锈
E、加快轴承转速
参考答案：A, B, C, D
人工授精受孕的最佳时间是（）。
A.排卵前后的3～4天
B.排卵前一周
C.排卵后一周
D.月经期末
E.月经前2～3天
It is _____ he often fails in exams _____ makes his parents worried about him.
A．what; that
B．that; what
C．that; that
D．/; that
