当超声束从一个介质穿到另一个弹性和密度都不同于前者的介质时，声束方向将发生变化，该变化叫

A．折射

B．稀疏

C．多普勒效应

D．反射

E．衍射

--- There is lots of water all over the floor. Would you please         _?

---With pleasure.

A．mop it up

B．mop up it

C．to mop it up

D．to mop up it