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

Rick often saw woodpeckers perched in trees in his backyard, but he never saw them at his bird feeders. Rick usually filled his feeders with sunflower seeds. He wondered if he could attract woodpeckers by adding a different type of food to the feeders.
To test this idea, Rick gathered eight identical bird feeders. He filled four with sunflower seeds. He filled the remaining four feeders with crushed peanuts. Rick then placed the bird feeders in his backyard. Over the next five days, he counted the number of woodpeckers that visited each bird feeder each morning.
Hint: An independent variable is a variable whose effect you are investigating. A dependent variable is a variable that you measure.
Figure: a woodpecker at a bird feeder.
Which of the following was an independent variable in this experiment?
A. the number of woodpeckers that visited the bird feeders
B. the type of food used in the bird feeders
Answer with the option's letter from the given choices directly.

B