对下列化学用语描述和理解都正确的是（ ）
A：原子结构示意图，可以表示12C，也表示14C
B：比例模型可以表示水分子，也可以表示二氧化碳分子
C：结构示意图为的阴离子都不能破坏水的电离平衡
D：电子式可以表示羟基，也可以表示氢氧根离子
阅读下面的短文，从所给的单词中选择合适的单词补全短文，每空只填一词，每词只准用一次，注意书写工整。
fly, flying, buy, take, taking, stay, bought, with, was, favourite, bored, fever
Sam ____ very sad last weekend. He was ill. He had a ____. His mother asked him to ____ in bed. He was very ____. Saturday afternoon, his good friends Tom and Mike came to see him. They ____a lot of fruit and some flowers for Sam. He was very happy.
This weekend, Sam is going to go hiking ____ his friends. They are going to fly kites, ____ pictures and play football. Because Sam likes____ kites. Tom's hobby is ____ pictures. And playing football is Mike's____ sport.
一个三角形中直角的个数最多有（ ）
提高环境承载力的直接方法有（ ）
