题目

---- Do you know that not until the winter of 2009 ______ to know each other?---- Yes, I know. It was not until the spring of 2011 that ______ married. A. they got; they got              B. they got; did they get   C. did they get; did they get    D. did they get; they got 答案：D在铁架台上固定一个由定滑轮和一个动滑轮组成的滑轮组，在绳子末端用竖直方向0.8N的拉力使2N重的物体匀速上升，下面说法正确的是（ ）A．滑轮组的机械效率是83.3%B．滑轮组的机械效率是66.7%C．绳端移动速度与物体上升速度相同D．绳端移动速度是物体上升速度的两倍