问答题 Do friends of the opposite gender distract teenagers,hampering their academic performance?It may seem obvious,and yet it is hard to prove.A survey of American schoolchildren conducted in 1995,for example,suggests no link between the proportion of a girl's friends who were boys and her grades.A new paper by Andrew Hill of the University of South Carolina,however,comes to a different result.He finds that for every 10%more children of the opposite sex among a student's friends,his or her grade-point average（GPA）declines by O.l（GPAs range from O t0 4）.Not only may having more friends of the opposite sex be bad for grades,but it has other effects,too.Among the children from the original survey who were successfully re-interviewed 14 years later,those whose grades had been dragged down by friends of the opposite gender were more likely to be married.