Which of the Following Situations Would Not Be a Likely

Which of the following situations would not be a likely candidate for the use of the joint method of agreement and difference?

A) Allergists suspect that some common substance is causing my rash; they decide to conduct experiments to track down the exact cause.
B) Epidemiologists suspect that a high intake of salt is related to heart disease so they decide to investigate the question.
C) Medical researchers want to know the cause of a nerve disease; they hope to study a population which has a high incidence of the disease to see if they can learn something about it and if possible develop a treatment and test it on the same population.
D) A dentist wants to know if her new denture linings are as good as she thinks, so she phones all of her denture-wearing patients, including those wearing other linings, and asks them how their denture linings are holding up.