Why do Aeroplanes fly at 35000 feet?

Related questions

1. Why did Concorde fly so high?
2. Do pilots sleep while flying?
3. What caused Air France crash?
4. Is Boeing 777 200 Safe?
5. Will the Concorde ever fly again?
6. Will a Concorde ever fly again?
7. How many Airbus a320s have crashed?
8. Why was Concorde scrapped?
9. How many planes are in the air right now?
10. How did Air France 447 Crash?

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