Imagine That a Study Has Just Been Published Which Shows

Imagine that a study has just been published which shows a relationship between a certain nutritional supplement and a lower incidence of cold and flu. This study was based on survey data and asked people to report whether or not they used this supplement in the last four months, as well as estimate the amount of times they were ill in that same period, and the average duration associated with those illnesses. Their results show that people who take the supplement report being ill less often and/or for less time. The company selling the supplement wants to claim that this is scientific proof their product works. Which of the following is not a possible alternative explanation to the idea that the supplement reduces frequency and/or duration of illness?

A) placebo effect
B) the third-variable problem
C) the bidirectionality problem
D) all of the above are plausible alternative explanations
E) none of the above are plausible alternative explanations