题干
. The reason for all the changes being made _______to us yet.
A：has not explained
B：has not been explained
C：did not explained
D：were not explained
答案(点此获取答案解析）
B
的小滑块静止于P点．已知滑块和半球形容器间的动摩擦因数为μ，滑块所受支持力大小为FN ，摩擦力大小为Ff ，弹力OP与水平方向的夹角为θ．下列关系正确的是（ ）