是a black animal 还是an?

是a black animal 还是an? an animal还是a animal an useful animal?还是a useful animal? an animal doctor还是a animal doctor animal是用a还是用an做单数 black eraser前加an 还是a 是a small black bear 还是 a black small bear? a horse is an animal 是What animal 还是animals? 改错：A fox is an useful animal. a story between an animal and human What a dog.和 What an animal.