“Plant growth is dependent on the amount of fertilizer applied.” That was the hypothesis Mel and Bill decided on for their science project. They planted identical seeds in the same potting mix, and they used identical pots for each plant. Each pot received the same amount of water, and all the pots were placed in the same location in the greenhouse. After three weeks, Mel and Bill were able to summarize their data. Based on the data collected, what would be the MOST appropriate research question for further study?

A) How much fertilizer, in grams, will kill the plants?
B) What happens when plants are given too much fertilizer?
C) What environmental conditions will result in the most plant growth?
D) What is the optimum amount of fertilizer for continued plant growth?