8. a vacuum tube can be used to

a. change alternating current into direct current

b. increase the strength of a signal

c. turn a current on or off

d. all of the above

9. a solid-state component with three layers of semiconductors is a

a. transistor

b. diode

c. vacuum tube

d. resistor

10. what is the primary use of transistors in a mobile phone?

a. store electric charge

b. maintain proper voltage

c. store data

d. amplify the phone's incoming signal