A dog is a useful a_____

A dog is a useful a_____ a dog is a useful animal 的 a dog 要怎么解释为什么不是the dog The dog is (a/an/the//) useful any more A dog is a useful a______填空使句子更完整 ______(Dog)dog is ______ useful animal. One rainy day a woman with a dog got on a bus,The dog is very big.It is a_ as big as a young horse.The f_____feet are very dirty _______horse is a useful animal . it is a dog It is a dog. It is ( ) a dog? that a dog is 英语种类问题cake is a kind of foodcake is a kind of foodan elephant is a useful animalthe dog is intelligent cats are very useful animal都表示种类么? What_you_?A_.There is a good stxie ha o A house is a useful animal 翻译 a useful John Smith is (an) honest man.这里面为什么用an?The dog is (a) useful animal.这句里面为什么用a?最好详细一点, O_____ ,it is a useful book _____computer is a useful machine.加冠词?