Một người đi từ A đến B rồi lại quay về A,thời gian cả đi cả về là 4 giờ 30 phút.Tính quãng đường AB,biết rằng vận tốc lúc đi là 12km/h,lúc về là 15 km/h.

For many years people believed that the cleverest animals after man were the chimpanzees. Now, however, there is a proof that dolphins may be even cleverest than these big apes. Although a dolphin lives in the sea, it is not a fish. It is a mammal. It is many way, therefore, like a human being Dolphins have simple language. There are able to talk to one another. It may be possible for man to learn how to talk to dolphins. but, this will not be easy because dolphin cannot hear the kind of sounds man can make. If man wants to talk to dolphins, therefore, he will have to make a third language which both he and the dolphins can understand. Dolphins are also very friendly toward man. They often follow ships: there are many stories about dolphins guiding ships through difficult and dangerous water. 

1.Which animals do people think may be the cleverest?
A.chimpanzees        B.dolphins       C.mammals      D.fish
2.What are dolphins like?
A.fish         B.animals       C.human beings      D.ships
3.Which of  the following sentences is NOT correct?
A.chimpanzees aren't the cleverest.
B.Man may learn how to talk to dolphins.
C.Dolphins often guide ships .
D.Dolphin is fish.