So here we have an example problem that wants us to choose one of the 4 potential answer options down below that best completes this statement, which says that the occurrence of turnover in lakes relies on the fact that blank. Now, of course, recall from our last lesson video that seasonal turnover or just turnover refers to the process of the vertical mixing of water layers in a lake or an ocean, specifically during the spring and fall seasons. And the vertical mixing process is actually driven by changes in water density, and those changes in water density are due to changes in the temperature of the water due to the seasons. And so, in terms of what the turnover process really truly relies on, it really is a density-driven process that's driven by the changes in water density. And so we're looking for an answer option that focuses on that aspect.

And so notice that answer option a says water is at its least dense at 4 degrees Celsius. However, recall that this is not going to be true. Instead, we said that water is at its most dense at 4 degrees Celsius, and notice that answer option b says that water is at its most dense at 4 degrees Celsius. And so this is looking like a potentially correct answer option, but let's check answer option c and d just to be sure. Now option c says nutrients sink to the bottom of a lake or ocean over time.

Now recall from our last lesson video that this is actually true. Nutrients do sink to the bottom of a lake or ocean over time. However, the turnover process does not rely on nutrients sinking to the bottom of a lake or ocean. The turnover process relies on the fact that density changes, the water's density changes as the seasons change. And so, option c, although it is a true statement in general, it's not what the turnover process relies on.

Now option d says, oxygen sinks to the bottom of a lake or ocean over time, but really this is not going to be the case. Instead, the oxygen remains towards the surface of the water and is depleted at the bottom of the water, and so, the oxygen does not sink to the bottom of the water. So this is just not going to be true. So the correct answer here is answer option b. Water is at its most dense at 4 degrees Celsius, and this is an important fact that the turnover process relies on because, once again, this turnover process is a density-driven process.

It's driven by changes in water density, so it does rely on the fact that water is at its most dense at 4 degrees Celsius and that it's at different densities at different temperatures. And so, option b is the correct answer, and I'll see you all in our next video.