I know a little bit about Italy as my wife and I ________ there several years ago.

A．are going

B．had been

C．went

D．have been

可燃气体爆炸极限下限低，表明（）。

A.危险性高，泄露出少量气体，就可能达到爆炸极限范围引起爆炸；

B.危险性低，不容易爆炸；

C.泄露大量气体不会爆炸；

D.是安全气体。