The right answer is the oceans.
Runoff is water from the precipitations (rains for example).
Some precipitation falls as snow and can accumulate as ice caps and glaciers. When spring arrives, the snow melts and the water runs off. Much of the precipitation returns to the oceans or infiltrates the soil.
Groundwater is an inestimable supply of drinking water for humanity, the residence time this water is variable depending on the depth.
A molecule of water will take several hundred, even thousands of years, to emerge from an underground water table and reach the streams and oceans.
