对室温下某试管中的饱和澄清石灰水进行下列操作，试管底部均会有白色固体，其中没有发生化学变化的是(  )

A．某同学用吸管向该试管中吹气

B．加入少量生石灰粉末

C．加入饱和碳酸钠溶液

D．将试管放入沸水中

(一) Today our knowledge of food and what it does for our bodies are far more advanced than that of the old times. Now we know about vitamins and how each kind of vitamin helps in the growth of a particular, the food that we eat will take care of our body and so there is no need to take any kinds of vitamins unless our doctor tells us that our bodies are short of something which can be supplied by them. Generally speaking, everything we eat does some good to our body. But if we eat too much of one kind of food and pay too little attention to others, we may have too much of one kind and not enough of others. Then we may be in trouble. We are often told that we must eat some meat at each meal in order to get the necessary proteins. That is only partly true, for proteins are not found only in meat. We can also get them from some vegetables. The best advice about what to eat is that we should eat all kinds of food but never too much of any.

The main topic of this passage is______

A．food and health

B．vitamins and proteins

C．meat, vegetables and proteins

D．food and vitamins