题目

— Your mother is very strict with you. —Yes, so _______ and so __________. A. is she; is my father      B. she is; my father is C. is she; my father is     D. she is; is my father   答案：D如图所示，物体的质量为m=2kg，两根轻细绳AB和AC一端连接于竖直墙上，另一端系于物体上，在物体上另施加一个方向与水平成θ=60°角的拉力，若要使绳都能伸直，最终AC绳与竖直墙垂直，AC与AB成θ=60°角，求拉力F的大小范围？