A：比4He少一个电子
B：比4He少一个质子
C：与4He的同分异构体
D：是4He的同位素
《沁园春•雪》中，下阕中“秦皇”指____，“汉武”指____，“唐宗”指____，“宋祖”指____，“成吉思汗”指____。
Our headmaster isn't in his office. I can't find him anywhere. Did you notice him ______ the office?
—____________. I'm so glad to help you because you are my best friend.
