l was the first one in our class --(work)in the rural area.

I was the first one in our class working in the rural area.
属于现在分词短语作定语,one和working之间是主动关系,所以用现在分词.如果用不定式,一般表示要去做的事情,或者表示目的,所以不定式不适合本题.