把3，5，6这三个数填入圆圈中，使每条边上三个数的和都是11。

已知正n边形的一个内角为135°，则边数n的值是（　　）

    In recent years advances in medical technology have made it possible for people to live longer than in the past. New medicines and machines are developing every day to extend life. However, some people, including some doctors, don't agree with these life extending measures, and they say that people should have the right(权利)t o die when they want, They say that the quality of life is as important as life itself, and that people should not be forced to go on living when conditions of life have become unbearable. They say that people should be allowed to die with dignity(尊严) and to decide when they want to die. Others don't agree and say that life under any conditions is better than death and that the duty of doctors is always to extend life as long as possible. And so the quarrel goes on and on without a definite(明确的) answer.

氢气、氧气、氮气、一氧化碳、二氧化碳、甲烷六种气体中，占空气体积分数最大的是（填化学式，以下相同）____；能使带火星木条复燃的是____；使用燃气热水器时，若通风不畅，容易产生一种有毒的气体是____；能使紫色石蕊试液变红色的是____；能使澄清石灰水变浑浊的是____。资源广、燃烧值高、无污染的新型能源是____充入食品包装袋中可以防腐的气体单质是____。天然气的主要成分是____。