The world is not hungry，but it’s thirsty.   1  while we say we are short of water.Why？Because about 97% of water on the earth is sea water which we can‘t drink or use for watering plants directly.Man can only drink and use the 3% of water that comes from rivers and lakes.   2  ，because some of it has been polluted.
　　Now more water is needed.The problem is how we can avoid a serious water shortage in the future.  3 Scientists are making study in this field.Today，In most large cities water is used only once and then runs to the sea or rivers.but it can be used again.Even if every large city reused its water，the water will still not be enough. 4 .
　　The sea seems to give us the best answer.There is a lot of water in the sea.All that needs to be get the salt out of the sea water.This will be expensive，but it has already been used in many parts of the world.   6 So you see，if we can find out a way，we’ll be in no danger of drying up.
从方框中所给的选项中选出能填入空白处的最佳选项，补全短文。其中有一项为多余选项。