题目

______ he realized it was too late to return home. A. Not until dark  B. It was until dark that  C. It was not until dark that  D. It was dark that 答案：C如图所示，小球用长为L的细绳悬于O点，使之在竖直平面内做圆周运动，过最低点时速度为v，则小球在最低点时，细绳的张力大小为__________。（小球质量为m）