A recent study showed that viral medications might be useful for fighting certain bacterial infections. Participants with gastrointestinal issues were assigned to either a placebo or viral medication for the first four weeks of the study, followed by an additional four weeks on the other treatment. Their symptoms showed significantly more improvement from the viral medication than the placebo. What type of study is this, and what is the explanatory variable? A. An observational study where symptom improvement is the explanatory variable B. An observational study where the type of treatment (viral or placebo) is the explanatory variable C. An experiment where symptom improvement is the explanatory variable D. An experiment where the type of treatment (viral or placebo) is the explanatory variable