Robert is interested in ________ astronomy and he wishes he could make a trip into ________ space one day.

A．the; the

B．an; the

C．an; /

D．/; /

旋流器工作时，其轴心必须形成空气柱，底流必定以（）排出。

A、柱状

B、圆状

C、辐射伞形

D、锥状