题目

I _______ for five minutes; why don’t they come? A. am calling      B. called      C. was calling      D. have been calling 答案：D 下图所示，闭合开关使螺线管通电，可以观察到左边弹簧________，右边弹簧________(选填“伸长”、“缩短”或“不变”)．