【正确答案】The lecture is about two methods to promote services in business.
The professor gives examples about two friends of his who had a painting business.
The friends' first method is to get a letter or other documents from satisfied customers.
This serves as a proof, or testimonial that the service is a good one.
The second method is to compare the condition of the customer before using the service and after. Photographs were shown to potential customers of the same apartment before and after painting, in order to encourage them to use the service.

【答案解析】[听力原文]
M: In this part of the course we've been considering how to promote services and products. To get customers, a company can't just say, hey-our service is great! Let's consider a couple of ways to increase business in our lecture.
The first method of promotion I will mention is the use of a testimonial. Back when I was in college, I had a pair of buddies who decided to start their own painting business to earn extra money. The problem, of course, was how to get customers. Soon after they started, my friends asked one customer to write a letter saying whether they had done a good job. That letter got them another project. And my friends were careful to get and keep letters from happy customers. Soon business boomed.
Another method my friends adopted was called comparison or-in this case-the "Before and After" approach. Let me tell you how it works. My friends showed potential customers two photographs. The first was of an apartment before they had repainted it. The second was of that same apartment after repainting. The point, naturally, was to let people see how much better and brighter the house looked after a paint job. Not surprisingly, this method was highly effective and drew a good many customers. The point here is that one look and see the difference. A picture's worth a thousand words and all that.

Alternative Wording(替换表达)
Synonyms (a)promote—market (b)business—firm
(c)satisfied—happy (d)testimonial—evidence, affidavit
(e)potential-possible (f)customer—client
Paraphrases 1. Two approaches about how to promote services are being discussed by the professor.
2. The first method involves obtaining a testimonial from a happy client.