如下图所示，点燃和熄灭酒精灯的操作中正确的是（　　）

A．1和3

B．1和4

C．2和3

D．2和4

It was only after I came across that matter_______ how important it was to master a foreign language.

A．did I realize

B．had I realized

C．that I realized

D．when I realized