Which of Earth's compounds makes the planet unique because it can exist in solid, liquid, or gaseous states under normal conditions?

A. water

B. methane

C. ammonia

D. ozone

A. water

Water is the unique compound on earth which can exist in solid, liquid and gaseous state under normal conditions. In normal circumstances, water can become solid ice when temperature on earth drops to 0 degrees celsius. At the same time, when water in liquid form is heated to 100 degree celsius, it changes into gas in the form of vapor. Thus, it can transition into all three states as temperature changes accordingly, and each of these temperatures do occur in planet earth