It's very friendly__ him to help me when I 'm in trouble.A for B to C of D with

It's very friendly__ him to help me when I 'm in trouble.A for B to C of D with
It's very friendly__ him to help me when I 'm in trouble.A for B to C of D with

It's very friendly__ him to help me when I 'm in trouble.A for B to C of D with
C
friendly修饰的就是人,所以用of,若是修饰事件的则用for
It's difficult for him to learn English well.

B
固定用法：对……友好 be friendly to sb
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选C of 是表示某人具有什么样的品质的，其他几个介词无此词义 那个B，somebody is friendly to sb 某人对某人友好

C 在我有困难的时候，他很友善的帮助我。