根据语篇内容，选择最佳选项。
①Water plays an important role in keeping you healthy. It keeps your body work properly. But do you know that too much water can poison (使……中毒) you?
②Water poisoning happens when people drink too much water. This lowers the level of sodium (钠) in their blood to dangerous levels.
③You probably already know that high levels of sodium can be unhealthy. That's true. But low levels are also harmful. When you have too little sodium in your body, your cells (细胞) begin to swell (膨胀). As a result, a lot of things will go seriously wrong in your body.
④Your brain may begin to swell. You may get a headache, feel very tired and want to throw up (呕吐). You may even lose consciousness (知觉). In these situations, water poisoning can make you dead.
⑤Protecting yourself from water poisoning may seem easy—just don't drink too much water, right? In some situations, however, this is easier said than done. For example, when you do some heavy exercise, such as running 40 kilometers, you may have to drink a lot of water. Then, it is important to balance (平衡) the amount of water you drink with the amount of water your body is losing. This can help you stay healthy whenever you are exercising.
⑥So how much water should a person drink each day? In fact, the actual amount needed is different for each person. Today, most doctors are telling people that they should drink water whenever they feel thirsty.
⑦We can't live without water. Neither can we live with too much water in our bodies. Make sure you take the right amount of it each day—staying healthy is all about staying balanced.
