A cubic foot of water at the ocean's surface weighs about the same as a cubic foot from the bottom of the Marianas Trench because?

A cubic foot of water at the ocean's surface weighs about the same as a cubic foot from the bottom of the Marianas Trench primarily because water is nearly incompressible. This property means that while there is an increase in pressure at greater depths, it does not significantly affect the volume of water, and thus its density remains relatively constant.

In essence, the average density of water at different depths in the ocean does not vary enough to result in a substantial difference in weight for equal volumes. While the deep ocean experiences pressure changes that might lead to minute differences in density, these do not create a significant overall effect on the weight of a cubic foot of water.

The other options focus on density variations or chemical composition, which may cause confusion. However, the key to understanding this question lies in recognizing water's incompressibility and its effective constancy in density across different ocean depths.