既是气体进出的通道又是食物经过的是通道是（　　）

There ______ a book, some boxes and some keys on the table.

Na₂SO₄是制造纸张、药品、染料稀释剂等的重要原料．某Na₂SO₄样品中含有少量CaCl₂和MgCl₂，实验室提纯Na₂SO₄的流程如图1所示

（1）加入Na₂CO₃溶液可除去的阳离子是____ （填离子符号），其反应的化学方程式为：____ ．若添加的NaOH溶液和Na₂CO₃溶液过量，可加入适量____ （填名称）除去．

（2）操作a的名称是____ ，该操作中需要用到的玻璃仪器有烧杯、玻璃棒、____ ．

（3）溶液M中的溶质是Na₂SO₄和杂质NaCl，观察图2中溶解度曲线，则操作b是：先将溶液M蒸发浓缩，再降温结晶、过滤．降温结晶的最佳温度范围是____ ℃以下，因为Na₂SO₄溶解度____ ．

根据文章内容，选择正确的答案。
   New York，London，Paris and other big cities are exciting places to live in. There are many interesting things to see and to go to. You can go to different kinds of museums，plays and films. You can also buy things from all over the world.
   But there are some problems in big cities too. It is expensive to live there and there are too many people in some places of big cities. Every year many people move to the cities to find jobs，to study at good schools.
   But sometimes these people can't find work or a good place to live in. Also it is hard to keep the cities safe(安全的）and clean.
   Some people enjoy living in big cities，others do not. Before moving to a big city，they should think about the problem of living there.

化学探究小组为测定某石灰石样品中碳酸钙的质量分数，取25g石灰石样品放在烧杯中，然后向其中逐滴加入一定量某质量分数的稀盐酸，使之与样品充分反应（杂质不参加反应）．随着反应进行，加入稀盐酸的质量与反应得到气体的质量变化关系如图所示．请完成下列计算内容：