Answering which question would best allow you to dig deeper into the content at the heart of the passage? Why does a fresh water body, when cooled, start freezing from the surface downward? Why does the behavior of fresh water cooling by convection change at 4°C? Can the temperature of the air above a fresh water body be subzero at the end of summer? Can ice form on the surface of fresh water if the lower layers are not frozen?
"Why does a fresh water body, when cooled, start freezing from the surface downward" would be the best question since this would explain all the difference processes.
