Explanation: A negative temperature coefficient for a crystal implies an inverse relationship between temperature and operating frequency. Specifically: * As the temperature *increases*, the crystal's operating frequency *decreases*. This aligns with option B. * Conversely, as the temperature *decreases*, the crystal's operating frequency *increases*. This aligns with option C. Both statements B and C accurately describe the behavior of a crystal with a negative temperature coefficient. Therefore, D, which combines B and C, is the correct answer. Option A describes the behavior of a crystal with a positive temperature coefficient. Crystal cuts like the X-cut are known for this characteristic frequency drift with temperature.