Final answer:
The heater cathode grid and two anodes in a CRT form a vacuum tube.
Step-by-step explanation:
The heater cathode grid and two anodes in a CRT (cathode ray tube) form a vacuum tube.
A CRT is a device commonly used in older TVs, computer screens, and x-ray machines. It works by accelerating electrons emitted from the cathode towards the anode, which excite atoms and molecules in the gas inside the tube causing them to glow and produce an image.
Therefore, the correct answer is c) A vacuum tube.