Serena is a research student who has conducted an experiment on the discoloration of marble. Read about Serena’s experiment. Then identify two flaws in her experiment’s design. Serena sees an article about the Taj Mahal, a medieval marble monument in India. She reads that the white monument is beginning to turn a dull yellow-brown. Serena admires the Taj Mahal and is eager to learn what might be causing the yellowing. She formulates a hypothesis that the yellow-brown tinge may be caused by air pollution from black carbon, brown carbon, or methane, combined with wind erosion.

To test her hypothesis, she decides to study the effects of these air pollutants and wind on white marble. She finds three identical pieces of white marble and exposes one of them to air containing black carbon, one to air with brown carbon, and one to air with methane. She also places a fan on two of the samples, varying the speed of the air stream in regular intervals. After a few days, the marble samples begin changing color, becoming grayish. Serena learns that air pollution caused the discoloration. However, she is not sure how it became gray instead of yellow and what exactly caused the discol