Researchers created a new fertilizer. They conducted an experiment to compare the new fertilizer to the old fertilizer. They divided tomato plants into two groups. They applied the new fertilizer to one group and the old fertilizer to another group. The variable is the number of tomatoes per plant on a randomly selected day. Which one of the following statements is valid?

a. The old formula of fertilizer was better because the plant with the most tomatoes was treated with the old fertilizer.
b. We can make no conclusions from this data about which fertilizer is better because the two groups have a different number of plants.
c. The average number of tomatoes per plant with the old fertilizer is higher than the average amount of tomatoes per plant with the new fertilizer.
d. The new formula of fertilizer works better. Plants with the new fertilizer had more tomatoes.