一条河中所有鱼可以看做一个生态系统．

我国形成酸雨的主要原因是

在实验室中模拟工业制取NaHCO₃的实验步骤如下：

第一步：连接好装置，检验气密性，在仪器内装入药品．

第二步：先让某一装置发生反应，直到产生的气体不能再在C中溶解，再通入另一装置中产生的气体，片刻后，C中出现固体．继续向C中通入两种气体，直到不再有固体产生．

第三步：分离C中所得的混合物，得到NaHCO₃固体．

第四步：向滤液中加入适量的某固体粉末，有NH₄Cl晶体析出．

某种商品若按标价的八折出售，可获利20%，若按标价出售，则可获利（   ）

    One of my favourite cartoons are Pleasant Goat and Big Big Wolf. I have seen many cartoons. Because Pleasant Goat and Big Big Wolf is my favourite. I like it because it's much modern than another cartoons. The story is about a clever goat. It is the hero of the story. It's clever and brave. It always finds ways help other goats run away from Big Big Wolf. It leads other goats fighting the Big Big Wolf. They protect each other and work hardly together and they are always successful. The cartoon is very interested. I can't always help to laugh. If you haven't saw it yet, please don't miss it.