听力原文：W: Your survey sounds very good. How many people filled it in? M: I gave out 120 copies and I got 70 back. W: That's a very high rate of return. Who did you give your questionnaires to? M: I gave a copy to every student in my residence hall and a few to friends from other colleges. W: Don't you think that this will influence your results? M: What do you mean? W: The people in your hall of residence will all be about the same age. They are all students, most of them studying similar subjects and from similar backgrounds. Therefore, it is likely that they will have similar opinions. Your results represent student opinion not public opinion. M: So how are you going to do your research? W: I'm going to interview my respondents in the shopping hall. What I'll do is to ask people if they have five minutes to spare to answer a few questions. If they agree I will ask them some multiple choice questions and tick off their answers on my sheet. That way I can select people of all ages and attitudes, so my sample people should be reasonably representative. M: Isn't it very difficult to ask meaningful questions using multiple choices? W: Yes, it is. I suppose your survey has the advantage of more detailed information. However, in most cases people won't bother to give answers that require too much effort on their part. The secret to writing a successful survey is to write simple choice questions that target the information you are looking for. Therefore, it is better to write a lot of short specific questions than longer general ones. M: So that's why it is taking you so long to write. W: Yeah, but I hope I will be ready to start interviewing at the coming weekend. (20)