试判断下述正确的是（）。

A.紊流切应力取决于断面的平均流速

B.紊流切应力仅与速度梯度有关

C.紊流切应力与流速分布有关

D.上述说法都不正确

—Do you always spend lots of time      playing computer games?

—No, but I spend much time         my homework.

A．in; on

B．on; on

C．on; in

D．in; in