Bubba is learning about saturated solutions and crystal formation by creating rock candy. He carefully adds two cups of sugar to 1.5 cups of boiling hot water to a 1 L container and stirs until the sugar is fully dissolved, about 3 minutes. He adds two drops of blue food coloring and stirs for 30 seconds. He hangs a string attached to a pencil into the container. He surrounds the container with towels so the mixture cools slowly. After three days, he removes the string with crystals on it and weighs the rock candy on a kitchen scale. He sees that a saturated solution allowed about 11 grams of sugar crystals to form on the string. What is needed to make this a valid controlled experiment?
A. A controlled variable
B. A dependent variable (also called responding variable)
C. An independent variable (also called a manipulated variable)
D. Scientific supplies like beakers.