21. The net charge of a neutral copper atom is

A. 0
B. +1
C. -1
D. +4
E. None of the above

22. Which one of the following statements is true for the voltage in a parallel RL circuit?

A. The voltage always has the same amplitude and phase for every part of the circuit
B. The total voltage is equal to the sum of the voltages across the resistance and inductance
C. The total voltage lags the total current by less than 90
D. The total voltages is less than the sum of the voltages across the resistance and inductance
E. None of the above

23. A low-pass filter:

A. passes lower-frequency signals and reduces higher-frequency signals
B. passes higher-frequency signals and reduces lower-frequency signals
C. has no effect on ac signals
D. reduces the level of all frequencies by a significant amount
E. None of the above

24. What is one important thing transistors do?

A. Emit light
B. Amplify weak signals
C. Regulate voltage
D. Rectify line voltage
E. None of the above

25. At room temperature an intrinsic silicon crystal acts approximately like

A. a battery
B. a conductor
C. an insulator
D. a piece of copper wire
E. None of the above