Question ID:PT50 S4 Q5 Passage:Motor oil serves to lubricate engines and thus retard engine wear. A study was conducted to assess the effectiveness of various brands of motor oil by using them in taxicabs over a 6,000-mile test period. All the oils did equally well in retarding wear on pistons and cylinders, the relevant parts of the engine. Hence, cheaper brands of oil are the best buys. Stem:Which one of the following, if true, most weakens the argument? Correct Answer Choice:EChoice A:Cheaper brands of motor oil are often used by knowledgeable automobile mechanics for their own cars. Choice B:Tests other than of the ability to reduce engine wear also can reliably gauge the quality of motor oil. Choice C:The lubricating properties of all motor oils deteriorate over time, and the rate of deterioration is accelerated by heat. Choice D:The engines of some individual cars that have had their oil changed every 3,000 miles, using only a certain brand of oil, have lasted an extraordinarily long time. Choice E:Ability to retard engine wear is not the only property of motor oil important to the running of an engine.