I could have turned to Mr. Smith for help, but he ______ too busy then.

A．is

B．was

C．should be

D．has been

圆柱螺旋线的形成是点在圆柱面上______运动的轨迹。

A．匀速直线

B．匀速圆周

C．匀速直线和圆周复合