Physical activity in your childhood,such as playing sports and exercising,can greatly benefit your health when you are older. Therefore,it is important to promote physical activity in childhood for one´s good health. The schoolyard is one place where children and adolescents can be encouraged to take part in physical activity. Thus,knowing how schoolyards are used by students may give us some helpful ideas to promote their physical activity. A study was conducted at four schools in Denmark in order to investigate how much different types of schoolyard areas were used and whether students were active or passive in those areas. In the study,schoolyard areas were classified and defined by their primary characteristics. Grass represented playing fields and natural green lawn areas,often used for soccer,but without any marked lines or goals. Multi-court referred to fenced areas on various surfaces,like artificial grass and rubber,designed for tennis and other such ball games. Natural represented areas with,for example,bushes,trees,and natural stones. Playground represented areas with play equipment,such as swings and slides on safe surfaces like sand. Solid Surf ace described the areas with the hardest surfaces,like concrete. These areas were identified by flat open spaces,often having numerous markings painted for games and benches set in different places. Using GPS devices and other instruments,the researchers measured the lengths of time the students spent in the different schoolyard areas as well as the degrees of their physical activity. Figure 1 displays the average amounts of time spent per day in each area for All students and those averages divided into Children （aged 12 and under） and Adolescents （aged 13 and over）. Solid Surface was clearly the area in which All students spent most of their time,followed by Multi-court then Grass. Natural and Playground showed similar averages for All students,with the average for All students in Playground being just over two minutes.
Figure 1. Average times spent in each area by All students,Children,and Adolescents. Furthermore,the study revealed differences between the average amounts of time spent in schoolyards by Children and Adolescents. In comparison with Adolescents,Children spent more time in all schoolyard areas except for Natural areas. The greater amount of time spent by Children might be explained by the fact that,according to the regulations at all four schools,Children could not leave the schoolyard during lunch time,but Adolescents could when they wanted to. When looking at the degree of physical activity,researchers discovered differences among the schoolyard areas. Students were most active in Grass and Playground areas. On the other hand,students were quite passive in Solid Surface areas,with Adolescents spending only 7% of their time there being physically active. The findings of this study show the importance of investigating the potential of various environments and features in schoolyards. To promote students´health,it is also beneficial to observe how varieties of games Children and Adolescents play affect the length of time spent taking part in physical activity. Let us now take a look at these relationships. （Henriette Bondo Ander sen 他（2015） Objectively Measured Differences in PhysicalActivity in Five Types of Schoolyard Area を参考に作成）
According to the passage,what is the difference between Multi-court and Solid Surface? ［ ］