A. The group of patients which is not treated with the new drug. They get either a placebo or the existing drug, so that their effectiveness can be compared with the effectiveness of the new drug.
B. The group of patients which is not treated with the new drug. They only get a placebo, so that the effect of the new drug can be examined.
C. A group of patients that will also get the new drug after the study is closed, to control the results of the study.
Answer 2: A
The control group is the group that will not get the new treatment (this is not necessarily the same as receiving a placebo). That should be the only significant difference between the control group and the study group (which follows the new treatment). Thus, in theory, the observed effect in the study group can only be due to the effect of the new drug and not to accidental or spontaneous changes.