A 10 kHz even-symmetric square wave is passed through a bandpass filter the centre frequency at 30 kHz and 3 dB passband of 6 kHz. The filter output is

• A. a highly attenuated square wave at 10 kHz
• B. nearly zero
• C. a nearly perfect cosine wave at 30 kHz
• D. a nearly perfect sine wave at 30 kHz