21. —What did you think of _____ president?

—I didn’t care for him at first, but after ______ time I got to like him.

A．the, a

B．/, the

C．the, the

D．/, a

超声波垂直入射到两种介质的界面上，如果两种介质的声阻抗很接近，声压的透过率将很大。