Here's a problem to solve: 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

Among the 4 following options, which is the correct answer?

- A: earthworms in the soil.
 
- B: fewer plants in the experiment.
 
- C: plants that received water without iron supplements.
 
- D: plants that received different amounts of sunlight.
 