Answer:
D. 2 hives exposed to low EMF levels, 2 hives exposed to medium EMF. levels, and 2 hives to high EMF levels
Explanation:
If you want to test if electromagnetic field cause CCD, the hives should be exposed to different levels because you want to check in general whats is the effect of it over the population. I would say the set up is still missing a control group which should not be exposed to magnetic field but live in similar conditions.
Answer B. provides a control group but the results obtain would be valid for only high intensity EMF.