earthworms in the soil. fewer plants in the experiment. plants that received water without iron supplements. plants that received different amounts of sunlight.
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: A student investigated the effect of iron on the growth of plants. Four identical plants received a different amount of iron supplement when being watered. This investigation could have been improved if there were
Possibilities:
- earthworms in the soil.
- fewer plants in the experiment.
- plants that received water without iron supplements.
- plants that received different amounts of sunlight.

plants that received water without iron supplements.