A mosquito bite can transmit to a person the parasite that causes malaria, and the use of mosquito nets over children' s beds can significantly reduce the incidence of malarial infection for children in areas where malaria is common. Yet public health officials are reluctant to recommend the use of mosquito nets over children' s beds in such areas. Which of the following, if true, would provide the strongest grounds for the public health officials' reluctance? ["Early exposure to malaria increases the body's resistance to it and results in a lesser likelihood of severe life-threatening episodes of malaria.", 'Although there are vaccines available for many childhood diseases, no vaccine has been developed that is effective against malaria.', 'Mosquito nets provide protection from some insect pests other than mosquitoes.', 'The pesticides that are most effective against mosquitoes in regions where malaria is common have significant detrimental effects on human health.']