题目内容：
Passage ThreeAmericans are well known for the strange diets they always seem to be following． It seems that Americans like to diet almost as much as they like to eat． New types of diet plans are always coming out． Usually, though, they don't stay popular for long．
There are many diets on the market． It is often difficult to know which ones really work． It's also hard to believe how fast a dieter is supposed to shed pounds． A lot has been written about dieting． And some interesting facts about diets and foods have been discovered．
For example, did you know that the more celery you eat, the more weight you will lose? Celery has “negative” calories． The body burns up more calories digesting a piece of celery than there are in the celery stick itself．
Dieters shun potatoes because they think they are fattening． But they aren't． A potato has about the same number of calories as an apple． To gain a single pound, you would have to eat eleven pounds of potatoes !
Some dieters even worry about getting fat from licking postage stamps． But they have nothing to worry about． The glue on an average stamp has only about one-tenth of a calorie． Maybe a diet of post- age stamps would be popular? This passage is all about______ A． vegetables
B． fads
C． Americans
D． dieting
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