Air pollution has caused millions of deaths every year,.____ ha

Air pollution has caused millions of deaths every year,.____ has become a great concern.
A、when
B、what
C、which
D、that
【正确答案】：C
【题目解析】：【考点点击】本题考查非限制性关系分句中关系词的选择。【要点透析】由逗号可知,分句为非限制性关系分句,而从句中又缺少主语,故用 which来引导,指代整个主句,故选C。