Why does price increase when demand increases?

Related questions

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2. Will the world ever run out of oil?
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4. What happens when prices high?
5. How does OPEC control the price of oil?
6. What happens when oil prices rise?
7. What was the price of oil in 2012?
8. Who decides crude oil price?
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Tags

1. #increase
2. #increasing
3. #price
4. #assuming
5. #supply
6. #decrease
7. #goods
8. #allows
9. #concept
10. #constant
11. #increases
12. #higher
13. #number
14. #always
15. #demand
16. #doesn