.

In _______ summer of 1963 Martin Luther King Jr. gave _______ speech to many black people.

A．不填; a

B．不填; the

C．the; the

D．the; a

离心风机主要由叶轮、机壳、（）、轴承传动件等组成。

A.集风器

B.批带盘

C.皮带

D.联轴器