A、Think about unpleasant images before sleep. B、Picture things they have done before sleep. C、Imagine worries and noises before

(19)The reason people count sheep, as opposed to bluebirds or sailboats, is uncertain: some authorities think it may have to do with a tallying system devised by shepherds in ancient Britain. But there is no question that the phrase has entered the language. And its meaning is clear enough—the sheer monotony(单调) of the task is meant to lull you to sleep. But does it work? Scientists at Oxford University put it to the test. In their study, which appeared in the journal of Behavior Research and Therapy, two sleep researchers recruited insomniacs and split them into groups. Then they monitored them as they tried different techniques for falling asleep on various nights. (20)What they found was that subjects took slightly longer to fall asleep on nights when they were instructed to distract themselves by counting sheep or were given no instructions at all. But when they were told to imagine a relaxing scene—a beach, for example—they fell asleep an average of 20 minutes sooner than they did on other nights. Counting sheep, the scientists suggested, may simply be too boring to do for very long, while images of a soothing shoreline or tranquil stream are engrossing enough to concentrate on. In other studies at Oxford, scientists compared "good" sleepers with insomniacs and found distinct differences in their pre-sleep thoughts. Insomniacs pictured less scenery of any kind and had more thoughts of unpleasant images, worries, noises in the environment, "intimate relationships" and things they had done during the day. (21)So don’t count sheep: instead, try picturing relaxing images.
19. Why has the phrase "count sheep" come into shape?
20. What does the test carried out by scientists at Oxford University show?
21. What does the speaker suggest to those who have difficulty falling asleep?