Six years ago, the police force of Harrisville was reduced, in an effort to make a smaller but more efficient force. In the first two years, crimes in Harrisville, especially by career criminals, were at a higher rate than they had been in the previous decades. Then, for the most recent four years, the crime rate has been the lowest in the seventy years on record, with the significant drop in crimes by career criminals. Which of the following, if true, most helps to explain the change in the crime rate between the first two years after the change and the last four years? ['The population of Harrisville increased slightly each year over the last six years, with a large number of senior citizens moving to a substantial retirement community within the city.', 'By the end of the second year of the newly organized police force, a large proportion of career criminals operating in Harrisville had been captured and incarcerated.', 'During each of the last six year period, retention and graduation rates at the local high schools have increased.', 'Of the seventy similar sized cities in this and neighboring states, 40% reported some drop in the crime rate over the last four years.']