静滑动摩擦系数的单位是（）。
A.kg
B.无单位
C.N²m
D.度
参考答案：B
There are different kinds of presents in the gift shop. I can’t decide _______.
A．to buy what
B．to buy which
C．what to buy
D．which to buy
直射光和散射光属于（）的光线性质
