—What would some students like to do after finishing their education?

—They would like to start to work ______ they needn't depend on their parents completely. [ ]

A. as soon as

B. so that

C. before

D. while

当声波穿过两种不同介质的界面时会发生折射是因为什么不同（）

A.声阻抗

B.声速

C.密度

D.传播路径

E.传播方式