【题目】Be-fore a problem can be solved, it must be obvious __________ the problem itself is.
A. what B. that C. which D. why
在线课程【答案】A
【解析】句意：在一个问题被解决 之前，必须弄清问题本身所在。it must be obvious __________ the problem itself is 中，it 作形式主语，__________ the problem itself is 从句 是真正的主语，从句中缺少表语，再根据引导词的意义可知选what。