Explanation: In a vacuum tube, the presence of residual gas molecules is indicated by a characteristic blue or purplish glow. This occurs when electrons, emitted from the cathode, collide with and ionize gas atoms within the tube. As these ionized gas atoms recombine with electrons or de-excite, they emit light, often appearing blue. This "gas glow" signifies a loss of vacuum integrity, commonly referred to as a "gassy" tube. A gassy tube's performance is degraded, it can suffer from internal arcing, and its lifespan is severely shortened. This blue glow is distinct from the normal dull red-orange glow of a heated filament. Therefore, options B (red glow) and C (yellow glow) are not indicators of unwanted gas within the tube.