先通读下面的短文,掌握其大意,然后在每小题所给的四个选项中,选出可以填入空白处的最佳选项。
You're thirsty and you get yourself a glass of water. Do you ever1how old that water really is? The glass of water that you're2may have fallen from the sky as rain only last week. However, water itself has been around pretty much as3as the earth has! In fact, 4oceans, seas and rivers cover 70％of the earth, there is a 5 supply(供给)of water, which keeps on moving round the earth. This is6of what's known as the water cycle. The sun heats up water and it turns into clouds, which are7from water vapor(水蒸气).When the clouds become8, the water falls back onto the earth as rain. Of course, clean water is absolutely essential for good health. The amount of safe drinking water has gone up around the world, but9one billion humans are still in great need10clean, drinkable water.
