Convection is a type of heat transfer happens when there is a movement of fluid (liquid or gas) in which convection currents are produced due to the movement of hot fluid upwards and cold fluid downwards. This happens as the density of hot fluid decreases due to presence of less number of mmolecules per unit volume. As the hot liquid becomes less dense, it starts moving upwards.
Thus convection occurs because heated material becomes less dense and rises.
D: Human feet come in a wide range of sizes. There isn't a lot of gray area or space for ranges of things in medel's laws, but they still work in a lot of cases.
