Julie: Look! The elephants are eating grass. They are so big.

Susan: They are big and friendly to people.

Julie: There are giraffes over there.

Susan: What are they doing?

Julie: They are playing with the elephants.

Susan: Do you like giraffes?

Julie: Yes, I do. They are very cute.

Susan: Are the koalas there sleeping?

Julie: Yes, they are. They usually sleep during the day.

Susan: Is the lion eating there?

Julie: No, it isn't. It's relaxing. It relaxes for 20 hours every day.

如图所示的是初中化学中的三个重要实验，请回答下列问题：

（1）实验A中，倾倒CO₂时观察到的现象是____ ．

（2）实验B中，测得空气中氧气体积分数小于$\frac{1}{5}$的原因可能是（只答一条）____ 

（3）实验C中，观察到铁丝剧烈燃烧，____，生成黑色固体，放出大量的热．发生反应的化学方程式为　____ 　．

已知 $\alpha \in R,\sin \alpha +2cos\alpha =\frac{\sqrt{10}}{2}$ ，则tan2α=（   ）