All the following statements are true. Which one makes it possible to learn of the existence of extrasolar planets without seeing the planets themselves?
A. Planets reflect visible light from their stars.
B. All the planets in a planetary system tend to orbit their star in the same direction and approximately the same plane.
C. Planets exert gravitational tugs on their stars that cause stars to orbit around the center of mass of their planetary systems.
D. Planets emit infrared light.
