Many provinces have been hit with smog. If unsolved the problem may lead to more serious

问题补充：

Many provinces have been hit with smog. If unsolved, the problem may lead to more serious results.

A. having left

B. to leave

C. left

D. leaving

答案：

A. having left

B. to leave

(答案→)C. left

D. leaving

解析：句意：很多省份都遭遇了雾霾，如果任由问题不解决，它会导致更严重的后果。这里if条件句是省略式，完整的是If the problem is left unsolved，…，如果主从句的主语一致的话，可以省略从句的主语和be动词，所以选C。