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