Answer:
D
Explanation:
A vacuum tube is a glass tube that creates the conditions of a vacuum to allow for electric current to pass through it.
Originally, vacuum tubes were used in electronics to amplify electric signals, however, today they have been replaced by more efficient systems, such as transistors.
A vacuum tube can change alternating current into direct current, it can turn a current on or off, and as stated above, it can increase the signal strength. However, it cannot change electrical energy into thermal energy, and that is why the answer is D.
