it's proving difficult___the men.1 for rescuing 2 to rescuing 3 to rescue 4 in rescuing选择哪一个,及分别说出每个为什么对与错,

it's proving difficult___the men.1 for rescuing 2 to rescuing 3 to rescue 4 in rescuing选择哪一个,及分别说出每个为什么对与错,
it's proving difficult___the men.1 for rescuing 2 to rescuing 3 to rescue 4 in rescuing
选择哪一个,及分别说出每个为什么对与错,

it's proving difficult___the men.1 for rescuing 2 to rescuing 3 to rescue 4 in rescuing选择哪一个,及分别说出每个为什么对与错,
3 to rescue 这是动词不定式 其他三项都是介词短语
it 是形式主语 只有动词不定式 能作真主语 其他三个选项 都不能作主语 故 错
如果选其他 三个选项 it 就指代不明