Question

These problems are quite hard for the students _______.

A. to work them out    B. working out    C. to work out    D. working them out

Answer

C

【解析】

句意：对于学生们来说这些问题很难做出来。to work them out动词不定式；working out计算出，动名词形式；to work out计算出，动词不定式；working them out把他们计算出，动名词形式。这句话中使用的结构是：主语+be+形容词+for sb. to do.，在该句型中句子的主语同时又是动词不定式的宾语,动词不定式使用主动语态结构.能用于该句型的形容词有: difficult; hard; easy; good; safe; nice等，故动词不定式中不能有代词them，故应选C。