Most high schools begin their day at around 7:30 a.m., which leaves many teenagers nodding off in the morning. In fact, at least 20 percent of high school students _39_ in class on a typical day. The problem: teenagers need a lot of _40_ —about nine hours each night, experts say. And most of them aren’t getting enough. To help sleepy teens, some school districts have tried 41 the opening of the high school day. Kyla Wahlstrom, an educational researcher, has been following school districts that changed their 42 . One district changed its start time from 7:20 a.m. to 8:40 a.m., and Wahlstrom says the students have benefited from the change. 【題組】39 (A) chat away (B) fall asleep (C) hang out (D) mess around