信心满满的做英语题目,没想到第一题就把我难住了!而且怎么想都不知道答案The research is so designed that once _____ nothing can be done to change it.A. begins B. having begunC. beginningD. begun帮帮忙告诉我选什么,顺便解释下我们选这个!抱拳感谢了!