Context: The passage below describes an experiment.

Preston and his neighbors wanted to attract more woodpeckers to their yards. They read that woodpeckers often eat sunflower seeds. The neighbors wanted to find out if filling their bird feeders with sunflower seeds would increase the number of woodpeckers that visited their yards.
So, four of the neighbors filled their feeders with sunflower seeds. Another four neighbors left their feeders empty. Every morning for a month, the neighbors counted the number of woodpeckers they saw in their yards.
Figure: a woodpecker at a bird feeder.
In this experiment, which were part of an experimental group?
A. the yards with feeders filled with sunflower seeds
B. the yards with empty feeders
From the given choices, pick the correct answer and respond by indicating the letter of the correct option.

A