Unusually large and intense forest fires swept the tropics in 1997. The tropics were quite susceptible to fire at that time because of the widespread drought caused by an unusually strong El Nino, an occasional global weather phenomenon. Many scientists believe the strength of the El Nino was enhanced by the global warming caused by air pollution. Which one of the following can be properly inferred from the information above? ['Forest fires in the tropics are generally larger and more intense than usual during a strong El Nino.', 'If the El Nino in 1997 had not been unusually strong, few if any large and intense forest fires would have swept the tropics in that year.', 'Air pollution was largely responsible for the size and intensity of the forest fires that swept the tropics in 1997.', 'If air pollution enhanced the strength of the El Nino in 1997, then it also contributed to the widespread drought in that year.']