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?
A The "only green tea" group serves as the control group.
B This study is an observational study.
C This study is double-blind.
D Only (A) and (C) are true.
E All of the above are true.