several digital scales. a wet paper towel. several identical plant seeds. different sizes of containers.
I gave my students this multiple choice question: A student hypothesizes that the amount of water a plant receives will affect how long it grows before reaching maturity. To test this hypothesis, the student will need

Only one answer is correct among these 4 choices:
- several digital scales.
- a wet paper towel.
- several identical plant seeds.
- different sizes of containers.

Could you tell me which one is correct?

several identical plant seeds.