Task: Question Answering
Input (English): 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  
 (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.
Output (English): C