每一个承载系统对任何的外来干扰都有一定的忍耐极限，当外来干扰超过此极限时，生态系统就会被损伤、破坏乃至瓦解。无论是自然生态系统，比如水环境、大气环境、土壤环境，还是城市区域、流域等都存在环境承载力的问题。据此并结合所学知识回答问题。

宝应县青少年活动中心组织一次少年跳绳比赛，各年龄组的参赛人数如下表：

则全体参赛选手年龄的中位数是____ 岁．

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    After Columbus discovered the New Continent(新大陆) and 1 back to his motherland, Spain, he was warmly 2. But not everyone felt  3 at Columbus' Success. Some people thought Columbus was only a 4 and what he had done could be done by any other seaman. One day, at a dinner, some self-important men did what they could to 5 Columbus unhappy, saying that he had only done the 6 thing in the world. Hearing this instead of jumping up angrily, Columbus took an 7 from a dish and asked,“Look, here, gentlemen, is there anyone here who can make this egg stand upright?” They tried one by one but none could. Then the egg 8  back to Columbus. He broke it a little, put the egg on the table and it stood upright. “What can be easier than this, gentlemen?” he said, “Now you will say it's so 9 that everybody can do it. Of course, because I have shown you now.” They looked at each other and talked 10.