Critical Reasoning (CR) | Re: At the Centers for Disease Control and Prevention, epidemiologists

At the Centers for Disease Control and Prevention, epidemiologists recently proved that sandwiches, even if left in refrigerators, can become infested with viruses that cause illness. They found that infestation usually occurs after sandwiches have been left for seven weeks. For that reason, people should discard leftover sandwiches at least once every seven weeks.

Which of the following, if true, would most weaken the conclusion provided?

a) The epidemiologists could not understand why infestation