----- My son was admitted to a key university last year.

----- Congratulations! His hard work finally ________,

A．paid for

B．paid off

C．paid back

D．paid attention

电镜下可见神经元轴突内有（）。

A.粗面内质网

B.微丝和微管

C.滑面内质网

D.突触小泡