...now,been a key element of scientific endeavor. 这里的what引导...答:what引导宾语从句,在此是连接代词。It signals a change是主谓宾结构,in后面加的是宾补,由what来引出是什么。宾语从句的连接代词主要有who, whom ,whose ,what ,whoever ,whomever ,whosever, whatever, whichever等. 连接代词一般指疑问,但what, whatever除了指疑问外,也可以指陈述. Do you ...