— I called you yesterday evening, but there was no answer.
— Oh , I’m sorry I _________ dinner at my friend’ s home.
A:have
B:had
C:was having
D:am having
如果把玻璃罩里的空气抽掉,里面成了____,我们就只看见闹钟的铃在动,却听不见声音。
计算下列各式:
(1)(﹣1.25)+(+5.25)
(2)(﹣7)+(﹣2)
如图所示,在外力作用下某质点运动的v﹣t图象为正弦曲线.从图中可以判断( )