题目

Mary's uncle made ______ possible for her to travel around the world. A . it B . that C . this D . one 答案：A图中的灯泡在通电时，灯泡能被点亮．请在下面横线上填写当开关K1、K2处于以下状态时，灯泡A、B的亮熄情况：（1）K1断开K2闭合时________．（2）K1闭合K2断开时________．（3）K1和K2都闭合时________．