Ben wants to test whether the material used to make a disposable cup affects the amount of thermal energy transferred to a person's hand. He investigates by carefully pouring the same volume of 90°C water into a foam cup and into a paper cup. Which additional step in the investigation will help Ben determine which material reduces thermal energy (heat) transfer to the hand of a person holding the cup?

A. Stir the water in each cup for 10 minutes before measuring its temperature.

B. Measure the outside surface temperature of each cup after 10 minutes.

C. Vary the amount of water held by each cup before measuring its temperature.