Offshore oil-drilling operations entail an unavoidable risk of an oil spill, but importing oil on tankers presently entails an even greater such risk per barrel of oil. Ttherefore, if we are to reduce the risk of an oil spill without curtailing our use of oil, we must invest more in offshore operations and import less oil on tankers. Which of the following, if true, most seriously weakens the argument above? ['Tankers can easily be redesigned so that their use entails less risk of an oil spill.', 'Offshore operations usually damage the ocean floor, but tankers rarely cause such damage.', 'Importing oil on tankers is currently less expensive than drilling for it offshore.', 'Oil spills caused by tankers have generally been more serious than those caused by offshore operations.']