I Rewrite the sentences so that they have same meaning as the original ones
11. They will show flying cars to the public when they finish it 
=> Flying cars .....
12. They will show flying cars to the public when they finish it

=> Solar panels.......
13. We will be using biogas for cooking and heating at this time next month.
=> Biogas......
14. People won't burn fossil fuels for energy in the future

=> Fossil fuels ...
15. New York has more billionaires than Tokyo ( Fewer)
16. A group of foreigners visited the slums last week, didn't they? ( Visit )
17. The countryside doesn't suffer as much pollution as the city . ( Less )
18. Does overpopulation cause a lot of social problems in this area? ( Causes )
19. Jakarta doesn't have as many skyscrapers as Shanghai (More)
20. Let's go to the school cafeteria ( about )
II Change the sentences inton the passive voice
1 they will build a hydropower station in the North of the country next year.
=> A hydropower station......
2 they sell these chemicals every where in my hometown.

=> These chemicals......
3 my children will turn off the lights when going to bed.
=> The lights......