题干
_____ a certain doubt among the workers as to whether he could finish the task in a short time．
A：It wa
B：There had
C：There existed
D：It had
答案(点此获取答案解析）
C
A：It wa
B：There had
C：There existed
D：It had
的轻质细杆
，
端固定一质量为
的小球，小球以
点为圆心在竖直平面内做圆周运动，通过最高点时，小球的速度为
，取
。则此时