Tom could swim very well at the age of seven.      [ ]

A. at he age of seven    

B. at his age of seven

C. when she was seven    

D. when he was seven

污染物在环境介质中通过分散作用得到稀释，不属于分散机理的是( )。

A．弥散作用
B．湍流扩散
C．物理扩散
D．弥散作用