C
非谓语动词做状语,选择哪种形式要看非谓语动词和句子主语之间的关系,以本题为例,句子主语是i, 非谓语动词是know, 我知道很多人需要.是主动的,所以用现在分词,相当于：Since I knew a lot of people needed medical help,i decided to be a volunteer worker.
过去分词作状语表示被动.试比较：1. Seeing from the top of the hill, we found our school looked beautiful. 2. Seen from the top of the hill, our school looked beautiful.
第一句主语是we, 我们去看,主动的用现在分词,第二句主语是school, 被看,所以是被动的用过去分词.
在中学阶段,不定式做状语一般只表示目的和出人意料的结果.例如：1. I got up early to catch the early bus. 2. I came back home only to find everything had been stolen.