According to "the great depression," why was the prosperity of the 1920s "unsustainable"?

Which of these is main idea? i need . (1) propaganda is information that is methodically spread in order to persuade audiences to adopt a certain opinion. (2) advertising is an ever-present form of propaganda in our lives. (3) four common propaganda techniques are present in the advertising we see and hear every day. (4) one technique, the testimonial, involves having a well-known person appear on behalf of the product being sold. (5) advertisers assume, for example, that if we admire a sports star, we’ll want to eat the cereal he or she endorses. (6) another common propaganda technique, the bandwagon, makes us want to be “one of the gang.” (7) “everybody’s switching to . .” “don’t be left out . . ” and “all across america, people are discovering . . ” are phrases that signal a bandwagon approach. (8) the plain-folks propaganda technique is especially popular on tv. (9) in plain-folks commercials, we see and hear “regular” consumers talk about their experience using a certain phone company, headache remedy, or brand of coffee. (10) the fourth common propaganda technique, the transfer, encourages us to link two unrelated objects in our mind. (11) when a powerful cougar prowls around a shiny new car, for example, advertisers hope we will transfer our sense of the wild cat’s speed, strength, and beauty to our vision of their product.