Discussion
The gravity measurements over the oceans indicate that generally the free-air anomaly is nearly zero. This is because :
Answer: B
The free-air anomaly is near zero over the oceans because ocean basins are in isostatic equilibrium.
The low density of seawater compared with rock is compensated at depth by denser mantle rising closer to the surface beneath the thin oceanic crust. Because the mass deficit of the water is balanced by the mass excess below, the total mass in each column is about the same as under the continents, and the free-air anomaly averages close to zero over broad areas.
The amount of sediment does not cause this balance, option C restates part of the idea vaguely without the compensation of the water, and the free-air anomaly is perfectly valid at sea.
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