The antarctic ice sheet in the southern hemisphere

holds almost two-thirds of earth’s fresh water
contains almost one-fourth of the world’s ice
is the only ice sheet remaining on the planet
holds almost two-thirds of earth’s salt water

The answer is it holds almost two-thirds of Earth’s fresh water. It also holds approximately 2% of the world's total water supply (this includes salty water) Antarctica is the largest ice sheet in the world, after the Arctic, which forms a whole continent. If Antarctica were to completely melt, the oceans levels would rise by approximately 60 meters.

i'm sure the answer you are looking for would be

a. an ice age

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