题目

I regret _____ hard at school, or I would have succeeded in passing the exam. A. not to work              B. having not worked      C. to have not worked     D. not having worked 答案：D21、下列属于生命现象的是（   ） A．火山爆发                             B．开水沸腾    C．雨后春笋                             D．潮起潮落