Mayor Pollister always repays his contributors by passing the laws they desire. Since Mayor Pollister recently passed the city' s first clean air act, he must have had a contributor to repay. Which one of the following is an assumption on which the argument depends? ['Mayor Pollister recently gained a great deal of support from a contributor.', 'Mayor Pollister would not have passed the law unless he was doing so for a contributor.', 'Mayor Pollister needed contributions to succeed in the election.', 'The city will be benefited by the passage of the clean air act.']