using different growing conditions for a group of the plants growing a group of tomato plants without fertilizer growing different types of plants with each fertilizer applying a combination of fertilizers to all the plants
I gave my students this multiple choice question: A group of students compared the effects of fertilizer on the growth of tomato plants. The students gave six tomato plants Fertilizer 1 and six other tomato plants Fertilizer 2. They grew the plants under the same conditions. After several weeks, the students concluded that the tomato plants that received Fertilizer 1 grew taller than those that received Fertilizer 2. Which of the following actions would have increased the accuracy of the results of this investigation?

Only one answer is correct among these 4 choices:
- using different growing conditions for a group of the plants
- growing a group of tomato plants without fertilizer
- growing different types of plants with each fertilizer
- applying a combination of fertilizers to all the plants

Could you tell me which one is correct?

growing a group of tomato plants without fertilizer