题目

－I called you yesterday evening, but there was no answer.     －Oh, I am sorry I _______ dinner at my friend’s home.     A. have          B. had            C. was having        D. have had 答案：C 如图所示，要使杠杆处于平衡状态，在A点分别作用的四个力中，最小的是 [　　] A． F1 B． F2 C． F3 D． F4