A year ago several regional hospitals attempted to reduce the number of patient injuries resulting from staff errors by implementing a plan to systematically record all such errors. The incidence of these injuries has substantially decreased at these hospitals since then. Clearly, the knowledge that their errors were being carefully monitored made the hospitals' staffs much more meticulous in carrying out their patient-care duties. Which one of the following, if true, most strengthens the argument? ['The incidence of patient injuries at a regional hospital that did not participate in the plan also decreased over the year in question.', 'The decrease in the incidence of the injuries did not begin at any hospital until the staff there became aware that the records were being closely analyzed.', 'The plan did not call for the recording of staff errors that could have caused patient injuries but did not.', "Under the plan, the hospitals' staff members who were found to have made errors that caused injuries to patients received only reprimands for their first errors."]