Can animals be made to work for us?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 ani-mals doing quite skillful things.Perhaps you have seen them on the television or in a film.If you?watch closely,you may notice that the trainer always gives the animal a piece of candy 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,and soldiers in both old and modern?times have used geese to give warning by making a lot of noise when a stranger or 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 nsed to watch out for faults in small steel balls that are?being made in one factory.When the pigeon sees a ball which looks different from others,it touches a?steel plate with its beak.This turns on a light to warn people in the factory.At the same time a few?seeds are given 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.Apes have been used in America in helping to make cars,and scientists believe that these large?monkeys may be one day gather crops and even drive trains.Many animals may be trained to do simple jobs if they know__________.
选项：

A:who their trainers are
B:they will be praised by their trainers
C:they will get a reward
D:something dangerous will happen to them