超声波在液体、固体中衰减很小,（）,尤其是在对光不透明的固体中,超声波能穿透十几米的长度,碰到杂质或分界面就会有显著的反射

A、a=1
B、a>10
C、a++
D、不确定

I sw womn running towrd me in the drk. Before I could recognize who she ws, she hd run bck in the direction ______ she hd come.of which B．from where C．in which D．from whichA．of whichB．from whereC．in whichD．from which

A、主运动只有一个
B、进给运动只有一个
C、主运动不止一个
D、进给运动和主运动都只能有一个