Why are many parts of the world facing scarcity of water?

Water demand has already been exceeding supply in many parts of the world. And as the world population continues to rise at an unprecedented and unsustainable rate, many more areas are facing water scarcity. Many regions are expected to experience this imbalance shortly. The world's supply of clean, fresh water is steadily decreasing and polluting. 

Therefore, the correct answer is an option (D).