Answer:
a. He is missing a control group of mice that did not receive the new antibiotic.
Explanation:
In any experiment especially experiments testing effectiveness of a drug, there should be a control group.
The importance of a control group is to minimize the effects of other variables other than the independent variable hence increasing reliability of the results.
in this case, a group of mice that did not receive the new antibiotic would have served as the control group.