下列有关化合价的叙述，正确的是（　　）

我们居住的星球之所以叫地球，你认为理由应该是（　　）

    Daniel comes from Sydney. He is now staying in Beijing with his family. He usually gets up at about 6:30 am and has breakfast at 7:00 am. Then he leaves home at 7:15 am.

    He gets to school at 7:45 am. His first class begins at 8:00 am. There are four lessons in the morning. Morning classes are over at 11:50 am, and he has lunch 10 minutes later.

    Afternoon classes begin at 1:30 pm. There are two lessons in the afternoon. They are over at 3:10 pm. Daniel usually stays at school for another hour to play soccer, and then goes home.