—Your bike was broken. Were you late for class this morning, Linda?
—No. Luckily, Mr. Green passed by and .
A:waited for me
B:let me down
C:kicked me off
D:gave me a lift
一个人骑自行车前进,开始以2m/s的速度匀速走了40m,然后又以5m/s的速度匀速走了50m,则他在整个路程中的平均速度是( )