Context: The passage below describes an experiment.

Justin 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.
Justin 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 an experimental group?
A. the cups that got cardboard sleeves
B. the cups that did not get cardboard sleeves
Select the correct answer from the provided options and reply with the letter associated with it.

A