题目

A river was running happily in the hill, the valley(山谷) and the field. Soon it was stopped by a huge rock and the river was a little afraid and worried what it could do.The rock said, “Hi, baby！Where are you going?”The river answered, “I’m going to the sea. My mother is waiting for me.”The rock made fun of the river and said, “You’re too young. Go back to the hill at once. You can’t get there. Don’t come back here again!”The river was surprised to hear that and said, “I can only go forwards(向前).There’s no river that goes backwards(向后)!”The rock became an下列物质分类错误的是（　　）A、H2-单质B、CO2-氧化物C、稀盐酸-纯净物D、KMnO4-化合物