A、To show how common microbes are. B、To prove how well his gardening methods work. C、To provide an example of an uncommon mold.

(20)This coffee can contains soil from my garden. And I prepared slide samples to show you that it is alive. This ordinary backyard dirt is crawling with microbes.(22)Microbes are not a very specific term. There are hundreds of thousands of different species called microbes. What they have in common is that we can’t see them with a naked eye. They are microscopic. Look at this slide. You should see some round cells. Those are yeasts.(21)Yeasts are fermenters. And they are necessary for making bread, beer, yogurt and so forth. Now look for an irregular shape with hairs coming out of it. That’s a mold. Molds are decomposers . and they are responsible for the decomposition mycoplastlees. You should also see some protozoa. Some protozoa, like the one causes malaria are harmful to people. A microbic that causes disease is called a pathogen. Finally you should see a lot of squiggly lines. They are bacteria—the oldest form of life on earth. Like these other microbes, bacteria are single-cells. But they are even simpler structurally because their cells do not have nuclear. So I’ve got about a teaspoon of soil in my hand here. In that teaspoon are about ten thousand protozoa, 200 thousand mold cells, a million yeasts, and probably a billion bacteria.
20. Why did the speaker take a soil sample from his garden?
21. What does the speaker imply about yeasts?
22. What does the speaker ask the class to look at during the talk?