In a vast ocean region, phosphorus levels have doubled in the past few decades due to agricultural runoff pouring out of a large river nearby. The phosphorus stimulates the growth of plankton near the ocean surface. Decaying plankton fall to the ocean floor, where bacteria devour them, consuming oxygen in the process. Due to the resulting oxygen depletion, few fish can survive in this region. Which one of the following can be properly inferred from the information above? ['The quantity of agricultural runoff pouring out of the river has doubled in the past few decades.', 'Before phosphorus levels doubled in the ocean region, most fish were able to survive in that region.', 'If agricultural runoff ceased pouring out of the river, there would be no bacteria on the ocean floor devouring decaying plankton.', 'The agricultural runoff pouring out of the river contributes to the growth of plankton near the ocean surface.']