首先it是代词，代替物、天气等。如 it is hot today 今天天气热
其次it可以作形式主语。如：It is important for us to eat healthy food
第三，which用于修饰定语从句如：wilma became the first American woman to win three Olympic gold medals in track,------made her mother very proud.A,it B,that C,which ,D,this该句重点是得判断是不是定语从句。
如果是就得用which。本句子中包含两个谓语动词，并且没有并列句引导词如and等，说明句子中包含从句，通过审题发现后一个分句缺少主语，说明是定语从句，所以答案为which。
主句主语用it，定语从句主语主语位置可以用which
