Context: The passage below describes an experiment.

Susan 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.
Susan 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, Susan weighed the amount of undecomposed food left in each container.
Figure: food decomposing in a compost bin.
In this experiment, which were part of a control group?
A. the containers that did not get worms
B. the containers that got worms
Determine the correct option from the provided choices and reply with its corresponding letter.
Answer: A