问题
选择题
Never before______ so highly successful in his attempts to forecast the weather accurately on a small scale．
A．man has been
B．man is
C．has man been
D．will be man
答案
答案：C
（考查时态和倒装句型。never置于句首时句子要倒装。never before用于现在完成时，因此答案为C。）
，质量相等的两个球分别固定在杆的A、B端，由水平位置自由释放，求轻杆转到竖直位置时两球的速度？ 