Plankton generally thrive in areas of the ocean with sufficient concentrations of certain nitrogen compounds near the surface, where plankton live. Nevertheless, some areas, though rich in these nitrogen compounds, have few plankton. These areas have particularly low concentrations of iron, and oceanographers hypothesize that this shortage of iron prevents plankton from thriving. However, an experimental release of iron compounds into one such area failed to produce a thriving plankton population, even though local iron concentrations increased immediately. Which of the following, if true, argues most strongly against concluding, on the basis of the information above, that the oceanographers' hypothesis is false? ['Certain areas of the ocean support an abundance of plankton despite having particularly low concentrations of iron.', 'A few days after the iron compounds were released, ocean currents displaced the iron-rich water from the surface.', 'The iron compounds released into the area occur naturally in areas of the ocean where plankton thrive.', 'Not all of the nitrogen compounds that are sometimes found in relatively high concentrations in the oceans are nutrients for plankton.']