Last summer, after a number of people got sick from eating locally caught anchovies, the coastal city of San Martin advised against eating such anchovies. The anchovies were apparently tainted with domoic acid, a harmful neurotoxin. However, a dramatic drop in the population of P. australis plankton to numbers more normal for local coastal waters indicates that it is once again safe to eat locally caught anchovies. Which one of the following, if true, would most help to explain why it is now safe to lift the advisory? ['Scientists have used P. australis plankton to obtain domoic acid in the laboratory.', 'A sharp decline in the population of P. australis is typically mirrored by a corresponding drop in the local anchovy population.', 'P. australis naturally produces domoic acid, though anchovies consume enough to become toxic only when the population of P. australis is extraordinarily large.', 'P. australis cannot survive in large numbers in seawater that does not contain significant quantities of domoic acid along with numerous other compounds.']