Answer:
(A) When the rate of ablation equals the rate of accumulation, the glacier retreats.
Explanation:
The glacier only retreats when the rate of ablation exceeds the rate of accumulation.
And the reason for the retreats and advancements is because of the regular variations in the shape of Earth's orbit and the configuration its axis sustained result to variations of solar intensity at high latitudes that the glacier retreats during warm summers and advance during cold summers.
And this is called the Milankovitch Cycles