A group of students wants to know what amount of a particular fertilizer will produce larger watermelons. Which of the following is the best choice to answer their question? Students should make observations of various fertilizers.

Students should conduct an investigation about various types of fertilizers.

Students should conduct a controlled experiment where they change the amount of fertilizer for watermelon plants and measure the difference in size of the watermelons.

Students should conduct both an investigation of various fertilizers and a controlled experiment where they change the amount of the particular fertilizer for watermelon plants and measure the differences.