Jeanette conducted an experiment to determine if plants grow better when exposed to rap music. She bought two identical plants, exposed one to rap music for two weeks, and let the other grow without music. Jeanette concluded that plants do not grow better with rap music. Which of these explains why Jeanette's conclusion may NOT be valid?

A. The experiment could never be repeated.
B. There was no control group.
C. She never formulated a question.
D. She should have tested more than two plants.