Context: The passage below describes an experiment.

Jenny has a bubble machine and wants to know how to make the bubbles last longer. She read that bubbles burst when the liquid that makes up the bubbles evaporates. Jenny knew that when liquids are warmer, they evaporate faster. So, she wondered if she could make her bubbles last longer by cooling the bubble solution.
Jenny cooled six bottles of bubble solution to 30°F below room temperature. She left another six bottles of bubble solution at room temperature. Then, she measured how long bubbles made from the solution in each bottle lasted.
Figure: bubbles at a party.
In this experiment, which were part of an experimental group?
A. the bottles that were cooled down
B. the bottles that were at room temperature
Select the correct answer from the provided options and reply with the letter associated with it.

A