Which number is main idea? . new technology often creates unanticipated problems. (2) automobiles, for example, provide numerous benefits, but they also pollute the air and kill about fifty thousand americans each year. (3) it is difficult to imagine life without electricity, but the generation of electricity pollutes the air and causes the thermal pollution of rivers. (4) insecticides and chemical fertilizers have performed miracles in agriculture but have polluted food and streams (and even “killed” some lakes). (5) jet planes, while us in many ways, cause air pollution (one jet taking off emits the same amount of hydrocarbon as the exhausts from ten thousand automobiles) and noise pollution near busy airports.