Beethoven was walking in the street _______ he heard someone ________ his music. [ ]

A. before, play

B. after, playing

C. when, playing

D. while, to play

污染物的扩散与下列因素有关的是( )。

A．大气扩散参数

B．下风距离

C．大气稳定度

D．取样时间

E．混合层高度