Well, Carlin pretty much has it right-there is not no blue food, but there's certainly not a lot of it. Fresh-picked blueberries are blue, though they become purple when they are turned into jam. The blue in blueberries-like the purple in grapes and red in tomatoes-is found in nature. But it isn't a hot color for food. People don't seem to prefer blue food.

Some diet programs even suggest that those determined to lose weight should make their food blue.

　　Eating, in part, begins with our eyes. Chares Spence-an experimental psychologist from Oxford University-points out that color can change our taste experience. Commonly, we consider red-colored foods up to 20% sweeter than they actually are; and green foods as being more sour. Spence suggests: human expectations may be influenced by our long history of watching-green fruits can become sweet as they grow up and turn red.

But what about blue? Except blueberries, much of the blue food we see these days is dyed blue artificially. Food producers argue that artificial color doesn't do much damage to health. A lot of research shows that some physical problems of kids are related to food dyes-while other studies show no effects at all. Blue birthday cake or even blue-dyed chicken can be served on the dinner table. So what?

21．George Carlin's words are placed at the beginning of the passage to _____.

A. show the author's opinion

B. tell a famous person

C. introduce a TV show

D. lead in the topic

22． Which of the following statements of blue is true?

A. Blue is not a very popular color for foods.

B. Blueberries are always blue, whether fresh or not.

The blue in blueberries is found unnatural.