Essayist: When the first prehistoric migrations of humans from Asia to North America took place, the small bands of new arrivals encountered many species of animals that would be extinct only 2, 000 years later. Since it is implausible that hunting by these small bands of humans could have had such an effect, and since disease-causing microorganisms not native to North America were undoubtedly borne by the new arrivals as well as by the animals that followed them, these microorganisms were probably the crucial factor that accounts for the extinctions. Which one of the following, if true, most weakens the essayist's argument? ['Very few species of North American animals not hunted by the new arrivals from Asia were extinct 2, 000 years after the first migrations.', 'Individual humans and animals can carry a disease-causing microorganism without themselves suffering from the disease.', 'Animals weakened by disease are not only less able to avoid hunters but are also less able to avoid their other predators.', 'Human beings generally have a substantial degree of biological immunity to the diseases carried by other species.']