—How long ago _________ you _________ the English Club?—I _________ it since two years ago.
A:did, join; joined
B:have, been in; joined
C:have, joined; have joined
D:did, join; have been in
父子两人沿周长为a的圆周骑自行车匀速行驶.同向行驶时父亲不时超过儿子,而反向行驶时相遇的频率增大为11倍.已知儿子的速度为v,则父亲的速度为( )
仔细观察图①,图中的“扶贫”更有利于实现( )