呼吸时体内进行气体交换的原理是（　　）

You'll feel much better if you_______ the pills.

如图中的物质为初中化学常见物质，他们之间的反应关系如图所示（“→”表示转化关系，“﹣”表示相互能反应，部分反应物、生成物或反应条件已略去）．甲与A溶液反应得到浅绿色溶液；A、B发生中和反应，产物之一是厨房内常用的调味品；C常用于改良酸性土壤，且C的溶液与D的溶液反应可得到B；C的溶液与乙的溶液反应生成蓝色沉淀；乙与丙两种溶液反应生成两种不同颜色的沉淀．请回答：

     One day, Bob took two of his friends into the mountains. They put up their tents(帐篷)and then rode off to a forest to see how the trees were growing.
      In the afternoon when they were about ten kilometers from their camp(营地), it started to snow. More and more snow fell. Soon Bob could hardly see his hand before his face. He could not find the road! Bob knew there were two roads. One road went to the camp, and the other went to his house, But all was white now. Everything was the same. How could he take his friends back to the camp?
Bob had an idea. The horses! Let the horses take them back! But what would happen if the horses took the road to his house? That would be a trip(行程) of thirty-five kilometers in such cold weather!
      It was getting late. They rode on and on. At last the horses stopped.
Where were they? None of them could tell. John looked around. What was that under the tree? It was one of their tents!

炎炎夏日，吃西瓜消暑是一件很惬意的事，请根据所学知识解答下列问题