问题补充：
They have had a long and bitter struggle their living conditions.
A. to improving
B. for improving
C. to improve
D. improve
答案：
A. to improving
B. for improving
(答案→)C. to improve
D. improve
解析：不定式常修饰动词状语，表示行为的目的、结果、原因等。for+动名词一般不做目的状语，但for+名词化了的动名词可作原因状语
