The 1988 drought in North America was probably caused by shifts in the temperature patterns of large equatorial stretches of the Pacific Ocean. The drought, ttherefore, is not evidence for the hypothesis that a long-term global warming trend, allegedly caused by atmospheric pollutants such as carbon dioxide, is occurring. Which of the following, if true, constitutes the best criticism of the argument above? ['There has been no warming trend in the United States over the last 100 years.', 'The consequences of global warming occur long after the actual emission of pollutants into the atmosphere.', 'A global warming trend could cause increases in the frequency and severity of shifts in temperature patterns in the Pacific Ocean.', 'Most pre-1988 droughts for which we have records were preceded by shifts in temperature patterns in the Pacific Ocean.']