【题目】_________ that causes heavy haze sweeping over china remains unknown to many scientists.
A. what it is B. what is it
C. who is it D. who it is
在线课程【答案】A
【解析】
试题分析：考察强调句。该句是强调句的疑问句，陈述句应该是it is … that cause heavy haze。又因为原因并没有查明是人还是物，所以疑问词用what。该句意思为：是什么导致了席卷中国的雾霾对于很多科学家来说还是未知的。