Can animals be made to work for us?Some scientists think that on

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 animals 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 modem 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 used 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. The trainer usually gives the animal a piece of candy or fruit__.
A、when it is hungry
B、before it plays a trick
C、when it starts to play a trick
D、after it has done the trick
【正确答案】：D
【题目解析】：根据文中描述，在动物完成某个技能或任务后，驯兽师会给它们一块糖果或水果作为奖励。这个过程被明确地描述在以下段落中：“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.” 这句话明确指出了奖励是在动物完成某个动作后给予的。 接下来，我们逐项考虑选项： A. when it is hungry - 文中并没有提到动物饥饿时会被给予奖励。 B. before it plays a trick - 文中提到奖励是在动物完成任务后给予的，而不是之前。 C. when it starts to play a trick - 同样，奖励是在任务完成后，而不是开始时。 D. after it has done the trick - 这与文中的描述一致，即在动物完成任务后给予奖励。 因此，正确答案是D。