Answer:
C or D I think
Step-by-step explanation:
D, you could compare frogs with no UV ray exposure to the ones with UV ray exposure
C, you can do the same but reverse
Edit: Meaning of control group is "A control group in a scientific experiment is a group separated from the rest of the experiment, where the independent variable being tested cannot influence the results. This isolates the independent variable's effects on the experiment and can help rule out alternative explanations of the experimental results."
So its more likely to be D