Why isn't the mass of a submerged submarine constant when it pumps air into its tanks?

The correct answer addresses how the mass distribution and buoyancy of the submarine are affected when it pumps air into its tanks. When a submarine pumps air into its ballast tanks, it effectively reduces the amount of water in the tanks and replaces it with air. Since air is less dense than water, this change not only alters the overall weight of the submarine but also influences its center of gravity.

As the air displaces the water, it may shift the center of gravity upward or in some other direction rather than keeping it constant. This movement can affect how the submarine balances and maneuvers underwater. The choice highlights the importance of understanding the mechanics of buoyancy and mass in relation to the submarine's design and operation.

In this context, the other options do not accurately capture the relationship between the submarine's weight and its buoyancy as it alters its ballast: the compression of air (which does not change its mass significantly) doesn’t impact the overall weight the way changing mass with water does, ignoring water weight when considering buoyancy does not fully explain observed changes in mass, and air being less dense than water does not clarify how the submarine’s mass—particularly as perceived by buoyancy—is affected during this process.