________Tom________ Mary speaks good Chinese, so they can talk with these Chinese students very well.
A:Not; but
B:Not only; but also
C:Both; and
D:Either; or
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( )