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Завершите предложения "должен" или "не должен"

  17 Pilotov Street
                                                                                    St. Peterburg
                                                                                    Russia
                                                                                    15/05/2008
Dear Donna,
I was glad to receive your letter. It’s really great that your parents let you have a pet.
As for me, I’ve got a kitten called Musik. It’s not difficult to look after him as I only need to clean his letter tray and to feed him. I think cats, dogs, hamsters and parrots make good pets because they are funny to match and it’s easy to take care of them.
Is your house big? Have you got a room of your own? Is there a garden around your house? If you say ‘yes’, you may have a dog or cat, Otherwise you’d better bay a hamster or a parrot as they don’t need so much space.
Well, I mast finish now аs I’ve got my aerobic class. Take care and stay in touch!
Love,
Mary

There is an opinion that ecological problems are the most serious ones today. I agree this ststement but I also think that there are other problems that are no less ecological ones. 
On the one hand, people can not live without wter and air but they contaminate them with different poisonous chemical, dropping litter and emitting gases from their automobiles. Moreover, ozon depletion leaves the Earth unprotected from radiation, which is dangerous for the human health. Besides, global warming is a serious problem because it can cause the melting of glaciers and water floods. 
On the other hand, people create themselves problems such as wars and global conflicts, making everybody suffer. To make the matters worse, there is danger of terrorism that we can not stop today and people themselvs become the victims of terror attacks. 
In addition, today there are a lot of undiaggnosed disease, which are dangerous for the whole population on our planet. 
Despite the existence of many problems, they can be solved. Many wars can be avoided with the help of negotiations and diplomacy. Talking of crime and terrorism.I think, that it is possible to find a solution with enforcing the law and talking to people of the other cultures. Having in mind dangerous diseases, I hope that scientists will invent new vaccines tu cure every disease and to prevent epidemics. 
To sum it all up, I would like to say that despite the existence of many issues,
ecological problems are still the most serious ones today and people should address these questions thinking about future generations. 

чень надеюсь, что