He wrote ________ song, which turned out to be rather popular with _______ young.

A．a; a

B．a; the

C．the; a

D．the; the

线型光束感烟火灾探测器的光束轴线至顶棚的垂直距离宜为（）。

A．0．1～0．5m 

B．0．1～1．0m 

C．0．3～0．5m 

D．0．3～1．0m