Context: The passage below describes an experiment.

Barbara wanted to set up a compost bin, where food could decompose, or break down, into soil fertilizer. She knew that worms can break down food. She wondered if adding worms to her compost bin would cause food to decompose faster.
Barbara set up four small compost containers. She added the same amounts of fruit peels, vegetable scraps, and eggshells to each container. Then, she added worms to two of the containers but not to the other two. After one month, Barbara weighed the amount of undecomposed food left in each container.
Figure: food decomposing in a compost bin.
In this experiment, which were part of an experimental group?
A. the containers that got worms
B. the containers that did not get worms
From the given choices, pick the correct answer and respond by indicating the letter of the correct option.

A