填空题语法预判技巧 — 英语词汇题

（雅思阅读填空题·语法预判）运用填空题语法预判技巧，先判断空格所需的词性，再从选项中选出唯一符合语法的词："Scientists propose that the decline in bee populations be treated as a serious ecological ______ that demands immediate ______." A. threaten / investigate B. threat / investigation C. threatening / investigative D. threatened / investigator