Answer:
I'd say B
Explanation:
answer B doesn't have any way to define what actually counts as "smart", so you wouldn't be able to test it.
answer A, you could run an experiment with fertilizer A on some plants, and no fertilizer on another.
answer C, you could give vitamin C supplements to some subjects, but not to others.
answer D, you could research or study elephants, and see if you find more than one species.
