A、Find the missing balls. B、Quickly identify the unusual steel balls. C、Pick up steel balls of good quality. D、Turn on a light b

Can animals be made to work for us?[32]Some scientists think that one day animals may be trained to do a number of simple jobs that are now done by human beings. They point out that at a circus, for example, we may see elephants, monkeys, dogs and other animals doing quite skilful things. Perhaps you have seen them on the television or in a film. If you watch closely,[35]you may notice that the trainer always gives the animals a lump of sugar or a piece of fruit as a reward. The scientists say that many different animals may be trained to do a number of simple jobs if they know they will get a reward for doing them.
    Of course, as we know, dogs can be used to guard a house,[33]and soldiers in both old and modern times have used geese to give warning by making a lot of noise when an enemy comes near. But it may be possible to train animals to work in factories. In Russia, for example, pigeons, which are birds with good eyesight, are being used to watch out for faults in small steel balls that are being made in one factory .[34]When the pigeon sees a ball which looks different from the others, it touches a steel plate with its beak. This turns on a light to warn the people in the factory. At the same time, a few seeds are given to the pigeon as a reward. It takes three to five weeks to train a pigeon to do this and one pigeon can inspect 3, 000 to 4, 000 balls an hour.
32. What do some scientists assume about animals?
33. What have people used geese to do according to the passage?
34. What can a trained pigeon do in a Russian factory?
35. How does a trainer usually reward the animal for its work according to the passage?