Context: The passage below describes an experiment.

Samuel drank coffee out of a paper cup. He remembered that his coffee shop had cardboard sleeves for their coffee cups. He wondered if using a sleeve would help keep the coffee warm.
Samuel placed a cardboard sleeve on each of three paper cups. He left three other cups without sleeves. Then, he poured the same amount of coffee into each of the six cups. He measured the temperature of the coffee in each cup every minute for ten minutes.
Figure: a coffee cup with a cardboard sleeve.
In this experiment, which were part of a control group?
A. the cups that did not get cardboard sleeves
B. the cups that got cardboard sleeves
Determine the correct option from the provided choices and reply with its corresponding letter.

A