A scientist sets up an experiment to test which fertilizer has the best effect on increasing fruit size in a single variety of avocado trees. They grow one group of avocado trees using Fertilizer-A, one group using Fertilizer-B, one group using Fertilizer-C, and one group grown without any fertilizer.

All the avocado trees are given the same amount of water and sunlight, and they are all grown during the same period of time.

Which of the following is the independent variable in this experiment?

The type of fertilizer used
The size of the avocados
The amount of water, sunlight, and time
The amount of water only