用数字和化学符号表示：

4个铝原子____；硫元素____；氦气____；

2个氧气分子____；5个钠离子____；

已知氮化镁中氮元素的化合价为﹣3价，请你写出氮化镁的化学式____

二氧化碳中碳元素的化合价为+4价____．

图一和图二分别表示A、B、C三种食物中一些营养物质含量的相对值，据图回答下列问题：

I liked to ____ (see) but I didn't like to hear.

    In 1620, about half the USA was covered by forests. Today the forests have almost gone. A lot of good land has gone with them, leaving only sand. China doesn't want to copy the USA's example. We're planting more and more trees. We've built the " Great Green Wall" of trees across northern part of our country. The Great Green Wall is 7,000 kilometres long, and between 400 and 1,700 kilometres wide. It will stop the wind from blowing the earth away. It will stop the sand from moving towards the rich farmland in the south. More "Great Green Walls" are needed. Trees must be grown all over the world. Great Green Walls will make the world better.
根据短文内容，选择正确答案。