When an apple is cut and exposed to air, its flesh turns brown. This reaction is controlled by an enzyme called polyphenol oxidase. The enzyme has binding sites for oxygen and phenol, a type of organic compound in apples. The list shows the sequence of events that results in a cut apple turning brown. Oxygen is introduced into damaged plant tissue.

The enzyme polyphenol oxidase binds oxygen from the air and phenols from the apple.

The enzyme catalyzes a reaction that changes phenols into o-quinones, a different organic molecule.

O-quinones react with amino acids in the apple's flesh to produce the brown color.

A student wants to investigate whether citric acid acts as an inhibitor of the enzyme polyphenol oxidase. Which experiment should the student use?

A.
Obtain a solution of o-quinone, add a small amount of phenol and citric acid solutions, and observe the color of the solution.
B.
Obtain a solution of polyphenol oxidase, add a small amount of citric acid, and observe the color of the solution.
C.
Place freshly cut apple slices in a dish, treat some slices with citric acid, and observe their appearance.
D.
Place freshly cut apple slices in a dish in an oxygen-free chamber, treat some slices with citric acid, and observe their appearance.