我国西北地区发展农业生产最重要的限制条件是下列的        （   ）

根据材料内容，选择正确答案。

    We see many animals, like rabbits(兔子), bees, dogs, birds, sheep……but do you know how these animals say things? First, let's see a rabbit. When a rabbit sees something dangerous, it runs away at once. When it runs, its tail(尾巴) moves up and down. When others see this, they run too.

    Many other animals use this kind of language. When a bee has found some food, it goes back to its home. It cannot tell other bees where the food is by speaking to them, but it does a little dance in the air. This may tell other bees where the food is.

    Some animals say things by making sounds like a man does. For example, a dog barks(吠) when a stranger(陌生人) comes near. Some birds can make several different sounds, each with its own meaning. In a word, every animal has its own language.

Mary didn't go to school yesterday. ____ ____ ____ she had a cold.

质量守恒定律是一条重要规律，请利用该规律的相关知识回答下列问题．

（1）化学反应前后肯定没有变化的是____ （填序号）．

①原子数目 ②分子数目 ③元素种类 ④物质种类 ⑤原子种类 ⑥物质的总质量

（2）镁条在空气中完全燃烧后生成氧化镁（只考虑氧气参加反应，下同），化学方程式为　____ ，若固体质量变化用图1表示，则m₂与m₁之差表示____ ．

（3）实验过程中，镁条点燃后实验现象如图2所示，镁条完全燃烧后，称量得知石棉网上留下固体质量反而比反应前的镁条还轻，其原因可能是____．