判断下面每组单词是否同类。

    We are learning English, but how can we learn English well ? A student can know a lot about English, but maybe he can't speak English.
If you want to know how to swim, you must get into the river. And if you want to be a football player, you must play football. So, you see, you can learn by using it. You must listen to your teacher in class. You must speak English to your classmates every day and also you must write something in English. Then one day you may find your English very good .

现有A、B、C、D  4种物质，其中A和B是由同种元素组成的化合物，C、D是气态单质。它们之间的关系如图1所示：