Which of the following statements presents a testable hypothesis? (A) More people watch basketball games than NHL games. (B) NASCAR is a popular sport in the Southeastern states. (C) Calcium is an essential nutrient for our bones. (D) If there is a relation between the wavelength of light and the photosynthesis rate, then light of different colors will cause the plant to make different amounts of oxygen. (E) Newton's law of gravitation explains the force that holds the solar system together.