In 2013, a medical study was conducted to determine if traditional Chinese medicine was effective in the treatment of diabetes mellitus. Eight hundred patients were randomly assigned to receive either traditional Chinese medicine only, a mixture of traditional Chinese medicine mixed with glibenclamide(the usual drug given for diabetes), or glibenclamide alone in two study groups: one group that had not received any drugs previously, and one group that had previously been treated with Metformin. The pills given to the patients were identical, and health workers were not informed which patients received which treatment. At the end of four years, it was found the traditional Chinese medicine was the most effective treatment, reducing the risk of hypoglycemia and improving glycemic control.

Using complete sentences, answer these questions: In this study,what is the sample? What is the control group? What is the explanatory variable? Is this an observational study or an experiment, and how can you tell?