在三角形，四边形中，具有稳定性的是____ ，举一个这类图形稳定性应用的实例____ ．

已知等差数列$\left\{a_n\right\}$的前$n$项和为$S_n,a_1=-11,a_5+a_6=-4,S_n$取得最小值时$n$ 的值为(  )

    When Mr. David retired(退休), he bought a small house in a village near the sea. He liked it and hoped to live a quiet life in it. But to his surprise, many visitors came to see his house in summer holidays, for it was the most interesting building in the village. From morning to night, there were visitors outside the house. They kept looking into the rooms through the windows and many of them even went into the house. He decided to drive the visitors away. So he put a notice on the window. The notice said，“If you want to satisfy your curiosity(好奇心), come in and look around. Price: twenty dollars.”Mr. David was sure that the visitors would stop coming, but he was wrong. More and more visitors came and Mr. David had to spend every day showing them around his house. “I came here to retire, not to work as a guide(导游)”, he said angrily. In the end, he sold the house and moved away.

下列日常生活中的变化现象，属于缓慢氧化的是（    ）

羰基硫(COS)可作为一种粮食熏蒸剂，能防止某些昆虫、线虫和真菌的危害。在恒容密闭容器中，将CO和H₂S混合加热并达到下列平衡： CO(g)+H₂S(g)COS(g)+H₂(g)    K=0.1，反应前CO物质的量为10mol，平衡后CO物质的量为8mol。下列说法正确的是（    ）