must, should, need; because, as for, because of
When I was a boy, I ______to take care of my young sister, ______my parents were busy with their work all day long.
【正确答案】：need / because
【题目解析】：Need作实义动词时，有人称和数的变化。Need to do sth. “需要做某事”，主语与do之间是主动关系。
