Why is groundwater an important source of freshwater storage?
a. it constitutes approximately 30% of earth’s freshwater.
b. it is the only viable source of freshwater in some areas.
c. it isavailable in conditions of drought.
d. all of the above

Answer is D.  All of the above.

Ground water is a water which is present under ground. Ground water accumulated in the pore space. Pore space is the space between dirt particles and rocks or water is stored in cracks and cervices of rocks. Different types of dirt particles and rocks store different amount of water.  Ground water is the main source of fresh water in some areas and it constitutes 30% Earth's fresh water. In case of drought, when surface water    is dried up ground water is reliable source of water.