听力原文： Let's turn our locus now to advertising. We all know what an advertisement is. It's essentially a message that announces something for sale. Now, there is an important precondition that must exist before you have advertising, and that's a large supply of consumer goods, that is, things to sell. You see, in the place with a demand for a product is greater than the supply, there's no need to advertise. Now, the early form. of advertising going back many hundreds of years with a simple sign there were shop doors that told you whether the shop was a bakery, a butcher shop or what have you, then was the advent of the printing crest. Advertising increased substantially as for products like coffee, tea, and chocolate appeared in newspapers and another periodicals as well as on the side of the buildings. In the American colonies, advertising in communication's media like newspapers and pamphlets became a major factor in marketing goods and services. By modern standards, these early advertisements were quite small and subdue, not the splash sheet whole page spreads of today, still some of them appeared on the front page of newspapers, probably because the news often consist of less and fresh reports from distant Europe, for the ads were current or local. Advertising really came into its own and became a central part of doing business, during the industrial revolution, suddenly there was a much greater supply of things to sell. And as we said earlier, that is the driving force behind advertising. People's attention had been drawn to the new product. Let's take a look at some of the advertisements from that time. What is the main topic of the passage?