Answer: D. downwelling
Downwelling is a oceanographic phenomena, in which the higher density material accumulates and sink below the low density water. The higher density sediments from the surface runoff sink down. The downwelling provides the supply of oxygen to the lower water layers along with the minerals present in the sediments. This allows the aerobic organisms to survive underwater. Therefore, Pedro should read the phenomena of downwelling to understand the fact that how aerobic organisms survive under water.