Concept:Cathode rays are streams of electrons emitted from the cathode in a discharge tube under specific electrical and pressure conditions.Explanation:A discharge tube requires a very low gas pressure so that gas molecules do not interfere too much with the electron flow.A high voltage is needed to accelerate the electrons emitted from the cathode strongly enough to produce visible cathode rays.Therefore, cathode rays are produced when the tube has low pressure and high voltage.Answer:D. Low pressure and high voltage