Context: The passage below describes an experiment. Read the passage and think about the variables that are described.

Duncan grew ten  on his back porch. He grew each plant in its own pot, and each pot was the same size and shape. Duncan noticed that his plants did not grow many leaves, and he wanted to see if earthworms could help them grow more. To test this idea, Duncan divided his ten plants into two equal groups. For one group, he added three earthworms to the soil in each pot. He did not add any earthworms to pots in the other group.
Two months later, Duncan counted the number of leaves on each of the ten plants. He compared the number of leaves on the plants in each group.
Hint: An independent variable is a variable whose effect you are investigating. A dependent variable is a variable that you measure.
Figure: mint plants.
Which of the following was an independent variable in this experiment?
A. the number of earthworms
B. the number of leaves
Answer with the option's letter from the given choices directly.
Answer: A