门机行走轮踏面磨损量应不超原厚度的（）%。
A.10
B.20
C.30
D.40
参考答案：B
You ______ be hungry already, you had lunch only two hours ago! [ ]
A. wouldn't
B. can't
C. mustn't
D. needn't
求下列不定积分：
