自由填空型

　　通读下面的短文，然后从后面各题所给A、B、C、D四个选项中选出可以填入空白处的最佳答案，并把答题卡上对应题目的答案标号涂黑。 One Sunday afternoon, my younger brother and I were left alone at home. I was doing my homework __11__ my younger brother was watching TV. Suddenly, the doorbell rang. My brother thought it was our parents, so he opened the __12__ quickly.