We can use phasor analysis to determine which of the circuits is a high-pass filter. A high-pass filter is defined as a circuit where the magnitude of the output signal and the magnitude of the input signal are identical when the input frequency goes to , and the magnitude of the output signal is when input frequency goes to .
The voltage gain (which means the phasor input voltage divided by the phasor output voltage) for the four circuits are For a high-pass filter, the magnitude of should go to as and to as . This is only true for and . The other two circuits are low-pass filters. Therefore, answer (D) is correct. |

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