Police commissioner: Last year our city experienced a 15 percent decrease in the rate of violent crime. At the beginning of that year a new mandatory sentencing law was enacted, which requires that all violent criminals serve time in prison. Since no other major policy changes were made last year, the drop in the crime rate must have been due to the new mandatory sentencing law. Which one of the following, if true, most seriously weakens the police commissioner's argument? ["Last year, the city's overall crime rate decreased by only 5 percent.", 'At the beginning of last year, the police department\'s definition of "violent crime" was broadened to include 2 crimes not previously classified as "violent. "', 'The city enacted a policy 2 years ago requiring that 100 new police officers be hired in each of the 3 subsequent years.', 'Studies of many other cities have shown a correlation between improving economic conditions and decreased crime rates.']