If Water Is So Abundant on Earth, Why Are Water

If water is so abundant on Earth, why are water resources such a big concern?

A) Water resources are not necessarily where humans need them and when they need them.
B) Humans are not very adept at exploiting water resources, so many go unused.
C) Most of the water on earth is frozen in polar ice sheets.
D) Most of the water is caught up in the hydrologic cycle and is therefore not readily available.