A transit company' s bus drivers are evaluated by supervisors riding with each driver. Drivers complain that this affects their performance, but because the supervisor' s presence affects every driver' s performance, those drivers performing best with a supervisor aboard will likely also be the best drivers under normal conditions. Which one of the following is an assumption on which the argument depends? ['The bus drivers are each affected in roughly the same way and to the same extent by the presence of the supervisor.', "The supervisors are excellent judges of a bus driver's performance.", 'For most bus drivers, the presence of a supervisor makes their performance slightly worse than it otherwise would be.', 'The bus drivers themselves are able to deliver accurate assessments of their driving performance.']