—What's your friend like?  

—_______.[ ]

A. He is fine.

B. He likes playing basketball.

C. He's friendly and helpful.

D. He's in Class Four.

可燃气体探测器探头电压信号的大小与可燃性气体浓度成（）关系。

A、正比例

B、反比例

C、函数增长

D、线性增长