Because the evaporation is like a filter it leaves most things behind like bacteria or salt then when it comes down from the sky (or rain :P) it is safe to drink (NEVER DRINK WATER THAT IS DRIPPING OFF A ROOF!!!). but if air pollution gets into the cloud it could contaminate the vapor and turn it into acid rain which can do lots of damage.
The correct answer is - aging population.
The aging population phenomena is becoming a big concern in the past few decades in some parts of the world. The most affected by this trend are the European countries, as well as Japan, the United States, Canada, Australia.
As the countries get more developed, and people earn more, it seems that they have much less children, usually one or two, and many people don't even have a child, and considering the upwards trajectory of the life expectancy, it results in an ever growing old population, and smaller and smaller young population.
This is a very dangerous trend on the long term as a nation may simply die out over time, as well as creating big economic issues.
Answer:
Champaran district in Bihar.
Explanation: