用所给单词的适当形式填空.
问题描述：
用所给单词的适当形式填空.
billion alone light air universe
1.There are four _____ in our classroom.
2.The girl is very shy,so she likes staying _____.
3.---How many galaxies are there in the _____?
---Sorry,I don't know.
4.The _____ in the morning is fresh,so you should get up early.
5.The rock(岩石) is about _____ of years old.
答
1.lights 2.alone 3.universe 4.air 5.billions