— When did you become a volunteer?

— Two years ago. I _____this group since then.[ ]

A. have joined in

B. have taken part in

C. have been in

D. have been to

黄土能保持高耸的直立陡壁是因为( )。

A．(A) 黄土中含有钙质结核

B．(B) 黄土强度高

C．(C) 具有垂直的节理

D．(D) 含有植物根系