题目

   The bed should be reserved as a place for sleep，but people tend to read an iPad a lot in bed before they go to sleep．    Charles Czeisler，a professor at Harvard Medical School，and his colleagues got a small group of people for an experiment．For five days in a row，the people read either a paper book or an iPad for four hours before sleep．Their sleep patterns were monitored all night．Before and after each trial period，the people took hourly blood tests to paint a day-long picture of just how much melatonin（褪黑激素）was in their blood at any given time如图，已知三角形木块ABC，∠A=30°，∠B=90°，AC=10cm，一只蚂蚁在AC、AB间往返爬行.当蚂蚁从木块AC边的中点O出发，爬行到AB边上任意一点P后,又爬回到AC边上的任意一点Q后,再爬行到点B，在这一过程中这只蚂蚁爬行的最短距离为 ．