The hottest place in the world is the Sahara Desert in Africa. But there are deserts in many other places: in America, in Asia and in Australia. In 1917 the temperature in Death Valley, in the United States, was over 48°C every day for six weeks. That's very hot!But deserts are not always hot. After dark, deserts are very cold.
   Not many animals and plants can live in a desert, because it doesn't rain very often. But one animal likes the desert — the camel. It has big feet to walk on the sand. It can live for a long time without water. Sometimes it doesn't drink anything for weeks — so it gets very thirsty! When it finds water it can drink 100 liters(升) at a time.

比较图中的a、b、c、d四个琼脂块中生长素的含量（）

某课外活动小组的同学们在教师的帮助下，探究用赤铁矿（主要成分是Fe₂O₃）炼铁的主要反应原理．他们设计的实验装置如图所示：

回答下列问题：

（1）写出用一氧化碳还原氧化铁的化学方程式____ ，装置A中所看到的现象是____ ．

（2）在C处看到的现象是____ ，说明反应后有____ 生成．

（3）本实验设计中，在B处所采取措施的目的是____ ．其反应的化学方程式为　____ ．

（4）在给氧化铁加热时，要先通人一氧化碳气体后加热，其目的是____ ．