Detailed Instructions: You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Problem: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.
Solution:
C