My family is very poor,so we have to live in a small ________.A.house B.building C.space room

My family is very poor,so we have to live in a small apartment为什么？house 大房子building建筑物space是指空间，比较抽象room是指房间apartment公寓为什么不是building？building主要指建筑物、大厦，如果用于居住，指一栋楼，或一单元比较多。