To test an herbal treatment for acid reflux disease, 100 volunteers who suffered from mild indigestion were randomly divided into two groups. Each person was given a month's supply of tea bags. For one group, the tea contained the herb mixed with green tea, whereas for the other group, the bags contained only the green tea. Participants were not told which type of tea they had, and were asked to drink one cup of tea per day for a month. At the end of the month, a researcher evaluated them to determine if their symptoms had improved. The researcher did not know which of the subjects had the tea with the herbal ingredient added. Which is true regarding this study? The "only green tea" group serves as the control group. This study is an observational study. This study is double-blind. Only (A) and (C) are true. All of the above are true.

In a study or research, the control group is the one that does not receive a treatment. This means, in this case, the control group includes those who only drink the green tea without the herbs because they serve as a standard to measure the effects of the herbs. This makes statement A (The "only green tea" group serves as the control group) true; also, the group receiving the herbs is the experimental group.

On the other hand, a double-blind study is one in which the researchers and the participants ignore who is receiving the treatment, which guarantees there is objectivity. This occurs in the study presented because neither the researcher nor the subjects known who received the herbal mix. Thus, statement C (This study is double-blind) is true.

Finally, a study is classified as observational if there is not intervention but the researchers just observe participants to study a phenomenon. This does not occur in the study presented because there is an intervention through the herbal treatment, and therefore the study is not observational.