The sustained massive use of pesticides in farming has two effects that are especially pernicious. First, it often kills off the pests' natural enemies in the area. Second, it often unintentionally gives rise to insecticide-resistant pests, since those insects that survive a particular insecticide will be the ones most resistant to it, and they are the ones left to breed. From the passage above, it can be properly inferred that the effectiveness of the sustained massive use of pesticides can be extended by doing which of the following, assuming that each is a realistic possibility? ['Periodically switching the type of insecticide used', 'Leaving a few fields fallow every year', 'Breeding higher-yielding varieties of crop plants', 'Gradually increasing the quantities of pesticides used']