Question

the passage below describes an experiment.
nina 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. nina 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.
nina 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.
in this experiment, which were part of a control group?
the bottles that were cooled down
the bottles that were at room temperature

Explanation:

In an experiment, the control group is the group that is not subjected to the independent variable (the factor being tested, which here is cooling the solution). The independent variable is cooling the solution, so the group not exposed to cooling (kept at room temperature) is the control group. The cooled bottles are the experimental group.

Answer:

the bottles that were at room temperature