一束平行光线经凸透镜折射后的传播方向如下图（a）所示，由图可知，此透镜的焦距大小约为________cm；若将一烛焰放在此凸透镜左侧2cm处，如图（b），则透过透镜可以看到一个_________ 、________ 的虚像。

I had just stepped out of the bathroom and was busy drying myself with a towel ________ I heard

the steps. [ ]

A. while  

B. when

C. since  

D. after