To test an herbal treatment for depression, 100 volunteers who suffered from mild depression 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 spice tea, whereas for the other group, the bags contained only the spice 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 psychologist evaluated them to determine if their mood had improved. The psychologist did not know which of the subjects had the tea with the herbal ingredient added.

Which is true regarding this study?

This study is an observational study.
This study is double-blind.
The "only spice tea" group serves as the control group.
All of the above are true.
Both (B) and (C) are true.