反馈控制是根据对系统未来的预测，事先采取措施应付即将发生的情况。( )
参考答案：错
— 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
简述油类污染表现在哪几个方面？
