An independent computer service company tallied the service requests it receives for individual brands of personal computers. It found that, after factoring in each brand' s market share, KRV brand computers had the largest proportion of service requests, whereas ProBit brand computers had the smallest proportion of service requests. Obviously, ProBit is the more reliable personal computer brand. Which one of the following, if true, most seriously weakens the argument? ['The company that did the tally receives more service requests for ProBit brand computers than does any other independent computer service company.', "The computer brands covered in the computer service company's tally differ greatly with respect to their market share.", "For some computer brands, but not for others, most service requests are made to the manufacturer's service department rather than to an independent service company.", 'ProBit has been selling personal computers for many more years than has KRV.']