地面铺轨前，应先铺设底砟，其厚度不小于（）。A.7cm B.8cm C.9cm D.

计算-2a（2a²+3a+1）的结果等于[ ]

A.-4a³-5a²+2a

B.-4a³+6a²+1

C.-4a³+6a²

D.以上都不对

He ______ the test again, in which case, his mother will be very disappointed. 

A．might fail

B．must have failed

C．should fail

D．could have failed

—Is the football under the chair?

—________. It's behind the door.

A. Yes

B. No

C. OK

D. No, I can't

超高频Si双极型晶体管的截止频率fT已达多少？

在“人体欲得劳动，但不当使极尔”中，“尔”的意义是（）

A.尽

B.完

C.语气词，罢了

D.指示代词，这里

E.以上都不是