单选题Questionsare based on the following passage.
　　Policemen, both in Britain and the United States, hardly recognize any likeness between their lives and what they see on TV.
　　The first difference is that a policeman's real life centers round the law. Most of his training is in criminal law. He has to know exactly what actions are crimes and what evidence can be used to prove them in court. He has to know nearly as much as a professional lawyer, and what is more, he has to apply it on his feet, running down an alley after someone he wants to talk to.
　　He will spend most of his working life typing millions of words on thousands of forms about　hundreds of sad, unimportant people who are guilty or of stupid, petty crimes.
　　Most television crime drama is about finding the criminal. In　real life, finding criminals is seldom much of a problem. Except in very serious cases like murders and terrorist attacks where failure to produce results reflects on the standing of the police little effort is spent on searching. The police have elaborate machinery which eventually shows up most wanted men.
　　Having made an arrest, a detective really starts to work. He has to prove his case in court and to do that he often has to gather a lot of different evidence. Much of this has to be given by people who don't want to get involved in a court case. So, as well as being overworked, a detective has to be out at all hours of the day and night interviewing his witnesses-and persuading them to help him.
　　A third big difference is the unpleasant moral twilight (衰落时期) in which the real one lives. Detectives are subject to two opposing pressures first, as members of a police force they always have to behave with absolute legality; secondly, as expensive public servants they have to get results. They can hardly ever do both.
　　If the detective has to deceive the world, the world often deceives him. Hardly anyone he meets tells him the truth. And this separation the detective feels between himself and the rest of the world is deepened by the simple mindedness as he sees it, of citizens, social workers, doctors, law-makers, and judges, who instead of stamping our crime, punish the criminals less severely in the hope that this will make them reform. The result, detectives feel, is that nine-tenths of their work is re-catching people who have stayed behind bars. This makes them rather cynical (愤世嫉俗).It is essential for a policeman to be trained in criminal law_________

A．so that he can justify his arrests in court
B．so that he can catch criminals in the streets
C．because many of the criminals he has to catch are dangerous
D．because he has to know nearly as much about law as a professional lawyer