To cut costs, a high school modified its airconditioning system to increase its efficiency. The modified system, however, caused the humidity in the school air to decrease by 18 percent. Twenty-four hours after the decrease in air humidity, a 25 percent increase in the number of visits to the school nurse was reported. This shows that a decrease in humidity can make people ill. The argument depends on assuming which one of the following? ['Most of the students at the high school suffered from the decrease in air humidity.', 'Modifying the air-conditioning system proved to be an ineffective way to cut costs.', 'It takes 24 hours after a person is infected with a virus for that person to exhibit symptoms.', 'At least some of the visits to the school nurse after the system was modified were due to illness.']