— I called you this time yesterday, but there was no answer.
— Oh, I'm sorry. I dinner at my friends' home.
A:have
B:had
C:was having
D:have had
第三次科技革命的特点不包括( )
m为在水平传送带上被传送的小物体(可视为质点),A为终端皮带轮,如图所示,已知皮带轮半径为r,传送带与皮带轮间不会打滑.当m可被水平抛出时,A轮每秒的转数最少是( )