Questions: Water in ocean currents can descend when it becomes colder and saltier than the surrounding water. If water descends to the bottom of the ocean, then other water must rise to the top. What causes water to rise to the top of the ocean? A. It cools down and dissolves additional salt. B. It becomes less dense than the surrounding water. C. It is forced upward by underwater mountain ranges. D. It is brought upward by organisms that live in the ocean.

The answer is the second one (B): It becomes less dense than the surrounding water.

Explanation for each option:

A. It cools down and dissolves additional salt.

• Incorrect. Cooling down and dissolving more salt would actually increase the water's density, causing it to sink rather than rise.

B. It becomes less dense than the surrounding water.

• Correct. When water becomes less dense, typically due to warming or a decrease in salinity, it rises to the top. This is a fundamental principle of fluid dynamics where less dense fluids rise above denser ones.

C. It is forced upward by underwater mountain ranges.

• Incorrect. While underwater topography can influence ocean currents, it is not the primary reason for water rising to the surface. The density difference is the main driver.

D. It is brought upward by organisms that live in the ocean.

• Incorrect. While some organisms can move water slightly, they do not have a significant impact on large-scale ocean currents or the movement of water to the surface.

In summary, water rises to the top of the ocean primarily because it becomes less dense than the surrounding water.