- Do you often hear John _______ in his room?

-Listen! Now we can hear him ________ in his room.[ ]

A. sing, to sing    

B. singing, singing    

C. sing, singing    

D. to sing, singing

当流体处于雷诺准数Re为2000～4000的范围时，流体的流动形态可能为湍流或层流，要视外界条件的影响而定，这种无固定型态的流动型态称为过渡流，可见过渡流是不定常流动。