IMSO 2005 Taiwan Selection — Science First Round Question 24

A student planted the same kind of seeds in pots A and B. She planted five seeds in pot A and five seeds in pot B. The amount and type of soil in both pots was the same. She added the same amount of water to both pots each week. Pot A was placed by a sunny window. Pot B was placed in a dark room. After four weeks, she observed the plants in both pots. The plants in pot A were green, with tall, thick stems. The plants in pot B were yellow, with tall, thin stems. Which statement best explains why the plants in pot A were different from the plants in pot B?