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