A、Patrolling the streets. B、Doing undercover work. C、Doing detective work. D、Supervising investigations. D男士称他从巡警做起，做到卧底，然后是侦探，而

W: Hi, Sam. Welcome to the "Ellen Show".
M: Thank you.
W: Sam, how long have you been a police officer?
M: I’ve been a police officer for thirty years.
W: Thirty years. And you’ve had different types of assignments on the police force, I guess.
M: Yeah, I’ve done everything from patrol to undercover work to detective work, and now I’m supervising investigations.
W: Sam, I think most people would say that being a police officer is a very stressful job. Would you agree?
M: Yes, it’s definitely a stressful job. But it depends on your assignment.
W: So, what’s probably the most stressful assignment you can have?
M: I’d say patrol is the most stressful assignment.
W: That’s interesting! In what way?
M: Well, I guess the biggest part of the stress is the fear factor—the fear of the unknown.
W: What do you mean, Sam?
M: Well, in patrol work, you don’t know from moment to moment who you are talking to or what their reaction is going to be to justify your presence. Let’s say, for example, a patrol officer stops someone for a traffic violation. It seems as though that would be a very low-stress situation.
W: Yes, it is a very low-stress situation.
M: But the truth is, there are more police officers injured during a routine stop.
W: Really?
M: Really! That’s why all police officers are taught from the very beginning to be aware of their surroundings. People back over policemen, people shoot policemen, people jump out at policemen—different kinds of things. So that’s probably the most stressful time.
W: I see. Well... let’s take a break and then we’ll move on to our next topic.
M: All right.
19. What is the present assignment of Sam?
20. Why does Sam say patrol is the most stressful assignment?
21. What should all police officers learn to do from the very beginning?