Context: The passage below describes an experiment.

Bob wanted to freeze soda to make popsicles. He knew that soda contains carbon dioxide gas, which forms bubbles as it escapes from the liquid. He wondered if the popsicles would freeze more quickly if he removed the carbon dioxide.
Bob took the lids off three small bottles of soda. He left the lids off overnight so that carbon dioxide could escape from the bottles. He kept the lids on three other bottles to keep the carbon dioxide in. The next morning, Bob placed all six bottles in the freezer. After two hours, he checked whether soda in any of the bottles had frozen.
Figure: soda with carbon dioxide bubbles.
In this experiment, which were part of a control group?
A. the bottles with lids taken off
B. the bottles with lids kept on
Select the correct answer from the given choices and respond with the letter of the chosen option.

B