Choose if the nouns are singular or plural

Some nouns are used only in the singular, even though they end in -s.

1. economics [singular|plural]
2. gymnastics [singular|plural]
3. headphones [singular|plural]
4. aerobics [singular|plural]
5. pyjamas [singular|plural]
6. trousers [singular|plural]
7. news [singular|plural]
8. clothes [singular|plural]
9. measles [singular|plural]
10. binoculars [singular|plural]
11. classics [singular|plural]
12. belongings [singular|plural]
13. stairs [singular|plural]
14. mathematics [singular|plural]
15. business [singular|plural]