某初级中学八年级（1）班同学为某期黑板报的出刊，收集整理了如下资料：“秋收起义”“井冈山会师”“红军长征”“西安事变”。据此你可以判断，本期黑板报的主题是（　　）

   There is a new shopping mall(商场) near my school. It is very big. There are five floors（层） of shops and each floor is very big. There are lots of clothes shops, but only one sports shop. There are lots of restaurants（饭馆） on the top（顶部的） floor, you can eat different kinds of food from different countries（国家）. I like going to see a movie. The mall is a really good place to go. It is a good place to meet friends.

已知,$\overrightarrow{a}=(2,1),\overrightarrow{a}·\overrightarrow{b}=10,\left|\overrightarrow{a}+\overrightarrow{b}\right|=5\sqrt{2},$则$\left|\overrightarrow{b}\right|=$（   ）

水变成水蒸气的过程中，发生变化的是（　　）

按要求完成下列小题：