题目

He's finished his composition，_________ he? A． isn't                   B．hasn't                         C．doesn't                   D．haven't 答案：B7．由同种元素组成的不同单质叫做同素异形体，下列各组物质属于同素异形体的是（　　）A．冰和干冰B．CO和CO2C．镁条和镁粉D．氧气与臭氧