Complete the sentences with the given words in their proper forms（用括号中所给单词的适当形式完成下列句子)  。

1. The students took the PISA test in _____subjects - reading, maths and science. (third)

2. Board games are more and more popular among _____. (teenager)

3. I liked to walk in fields on _____days and to see my footprints left behind. (snow)

4. _____people often begin their conversations by talking about the weather. (Britain)

5. The news about education has attracted the public attention _____. (recent)

6. The World Chocolate Park showed a_____of many kinds of chocolates. (collect)

7. To improve your writing skills, you'd better _____a habit of reading. (development)

8. Don't tell lies. Nobody likes those _____people. (honest)

天然气在空气中的浓度在（）时，遇明火可产生爆燃。因此，必须防止（）、（）、（）这三个因素的同时存在。