A、By door-to-door advertising. B、By using symbols. C、By verbal announcements. D、By written messages. C推断题。文中提到，最初工匠们对路人进行叫卖，告诉他们

Advertising is the tool that has always been used to convince the public to buy products. In the beginning, it was a basic and crude tool.[23]Craftsmen cried out to passersby, telling the quality of their products. Today, newspapers, magazines, radio, and television use a variety of means to introduce the public to many products developed through technology.
    The Romans started using recorded advertisements. They smoothed and whitened areas on a wall where advertisements could be written or carved.
    In the Middle Ages, people continued to use verbal announcements and written messages, but a new form of advertising, using symbols, was developed as well. Shops displayed a special symbol to indicate what goods or services could be found inside.
    The Industrial Revolution caused an explosion in the advertising field. The abundance of luxury goods, coming both from new inventions and from trade with different parts of the world, meant that consumers had to be told more about products than ever before. They had to be persuaded that they needed all these new products and that one product was superior to its many competitors. The buying public was soon being exposed to endless amounts of advertising.
22. What is the passage mainly about?
23. How did early craftsmen advertise their products?
24. Who first started using written advertisements?
25. When did advertising first boom?