题目

 When you refer to an English dictionary, you’re _____exposed to the basic words and learn how to use them.        A. merely                  B. repeatedly            C. occasionally               D. hardly 答案：B在空气和某种介质的界面上，反射现象和折射现象同时存在，入射光线、反射光线和折射光线相汇交于O点，如图所示．则这种介质的折射率可能是 [　　] A．B． C．D．