Several people came down with an illness caused by a type of bacteria in seafood. Health officials traced the history of each person who became ill to the same restaurant and date. Careful testing showed that most people who ate seafood at the restaurant on that date had not come in contact with the bacteria in question. Despite this finding, health officials remained confident that contaminated seafood from this restaurant caused the cases of illness. Which one of the following, if true, most helps to resolve the apparent discrepancy indicated above? ['All and only those who ate contaminated seafood at the restaurant on that date were allergic to the monosodium glutamate in a sauce that they used.', 'Those made ill by the bacteria had all been served by a waiter who subsequently became ill.', 'All and only those who ate a particular seafood dish at the restaurant contracted the illness.', 'The restaurant in question had recently been given a warning about violations of health regulations.']