Final answer:
The correct answer is D, indicating that vertical pressure gradients are much greater than horizontal pressure gradients, largely due to gravity's impact on fluid pressure with depth.
Step-by-step explanation:
Among the provided options, D.Vertical pressure gradients are very much greater than changes in horizontal pressure is true. In fluids, the vertical pressure gradient is greatly affected by gravity, leading to a substantial change in pressure with depth (p = hρg where p is the pressure, h is the height, ρ is the fluid density, and g is the gravitational acceleration). This is why atmospheric pressure decreases rapidly with altitude, as there is less atmospheric mass above a given level as one goes higher. In contrast, horizontal pressure gradients are typically much smaller because they are not directly influenced by gravity but by differences in the fluid flow.
When considering the flow of fluids, the pressure difference is the driving force, and flow occurs from a region of high pressure to a region of lower pressure. This principle also explains why atmospheric pressure can exert such a large force on us without us being crushed, as the pressure is exerted uniformly over our bodies, and our internal pressures adjust to balance this external pressure. This can also be seen in the way sandbags around a levee can hold back water, as the pressure exerted by the water within the sandbags balances the pressure of the larger body of water.