Charles knows that adding salt to water causes the freezing point of water to be lowered, but he wants to know if rock salt affects the freezing point more than table salt. Charles performed an investigation in his classroom in which he measured the freezing point of 250 mL of water. He used this measurement as his control. Then, he dissolved equal amounts of rock salt and table salt in two identical 250 mL beakers of water. Finally, he measured the freezing points of each beaker of water, analyzed his results, and formed a conclusion.

What type of investigation did Charles perform?