题目

There is a joke among flu researchers: “If you've seen one flu season, you've seen one flu season.” The joke is about the unpredictable nature of the flu virus．Every year it looks different, and every strain (类型) follows its own pattern — it's the reason why new strains like H1N1 are extremely difficult to predict.        Dr．Michael Osterholm is a former adviser to the U.S．Department of Health and Human Services．“I know less about influenza today than I did 10 years ago,” he says in a joking way．“Every stone we've turned over, we get more questions.”     �扇形的面积一定，问它的中心角α取何值时，扇形的周长L最小?