第6部分：完形填空
阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案。
Young Adults Who Exercise Get Higher IQ Scores
Young adults who are fit have a higher IQ and are more -l- to go on to university，reveals a major new study carried out at the Sahlgrenska Academy and Sahlgrenska University Hospital．
The results were recently published in the Proceedings of the National Academy of Sciences （PNAS）． The study involved l.2 million Swedish men doing military service who were born between l 950 and l976．The research group analysed the -2- of both physical and IQ tests the youngsters took fight after they started serving the army．
The study shows a clear link -l- good physical fitness and better results for the IQ test．The strongest links are for -4- thinking and verbal comprehension．But it is only fitness that plays a -5- in the results for the IQ test，and not strength．"Being fit means that you also have good heart and lung -6- and that your brain gets plenty of -7-，"says Michael Nilsson，professor at the S ahlgrenska Academy and chief physician at the Sahlgrenska University Hospital．"This may be one of the reasons -8- we can see a clear link with fitness，but not with muscular -9-．We are also seeing that there are growth factors that are important．"
By analysing data for twins，the researchers have been -10- to determine that it is primarily environmental factors and not genes that explain the link between fitness and a -11- IQ．
"We have also shown that those youngsters who -12- their physical fitness between ages of 15 and 18 increase their cognitive performance," says Mafia Aberg，researcher at the Sahlgrenska Academy and physician at Aby health centre．"This being the case，physical -13- is a subject that has an important place in schools，and is an absolute must if we want to do well in maths and other theoretical subjects."
The researchers have also compared the results from fitness tests -14-national service with the socio-economic status of the men later in -15-．Those who were fit at l 8 were more likely to go into higher education，and many secured more qualified jobs．
