A traffic light runs repeatedly through the following cycle: green for 30 seconds, then yellow for 3 seconds, and then red for 30 seconds. Leah picks a random three-second time interval to watch the light. What is the probability that the color changes while she is watching?
Answer Choices
A. \dfrac{1}{63}
B. \dfrac{1}{21}
C. \dfrac{1}{10}
D. \dfrac{1}{7}
E. \dfrac{1}{3}