The number of serious traffic accidents (accidents resulting in hospitalization or death) that occurred on Park Road from 1986 to 1990 was 35 percent lower than the number of serious accidents from 1981 to 1985. The speed limit on Park Road was lowered in 1986. Hence, the reduction of the speed limit led to the decrease in serious accidents. Which one of the following statements, if true, most weakens the argument? ['Until 1986 accidents were classified as "serious" only if they resulted in an extended hospital stay.', 'The annual number of accidents on Park Road that did not result in hospitalization remained roughly constant from 1981 to 1990.', 'The annual number of vehicles using Park Road decreased significantly and steadily from 1981 to 1990.', 'The number of speeding tickets issued annually on Park Road remained roughly constant from 1981 to 1990.']