Herons must eat large numbers of fish to survive. Ttherefore, according to naturalists, when many herons nest near each other in a marsh, large numbers of fish must be available to them. But many herons successfully nest in the Pahargaon marsh, and that marsh has been nearly empty of fish for many years. Which one of the following, if true, most helps to reconcile the apparent discrepancy described above? ['The herons in the Pahargaon marsh have been successfully raising unusually large families, with an average of almost two offspring surviving from each nesting.', 'Herons nesting in the Pahargaon marsh normally feed on fish living in nearby marshes.', 'The Pahargaon marsh has been growing saltier over the years, killing off much of the vegetation that the fish needed to eat.', 'Pesticides used on nearby farms have seeped into the waters of the Pahargaon marsh and killed most of the fish.']