题目

 --- Tommy, your mum has been out without glasses!  --- Well, take it easy. _______ she is back home for them.        A. It is long before that                              B. It won’t be long since        C. It won’t be long before                         D. It will be long before 答案：C一个水平放着的圆柱形水管的截面如图所示，如果水管直径为40cm，水面的高为10cm，那么水面宽AB=    cm，（不取近似值）．