A). 600,000 Hz or 600 KHz Yes. Commercial broadcasters operate here. This is the '600' on your AM radio dial.
B). 60 Hz No. In principle, this frequency might be used for communication or commercial broadcasting, but it suffers from two inconvenient truths: -- An efficient antenna for 60 Hz ... either transmitting or receiving ... needs to be almost 780 miles long. -- This is the frequency of the electric power utility in the US and Canada, so every outlet, wire, cable, lamp cord, and electric line on a pole RADIATES a little bit of signal at this frequency. That's an awful lot of interference.
C). 6,000,000 Hz or 6 MHz There's a lot of broadcasting activity here, but it's not commercial music, news, and sports into local homes and cars. It's foreign short-wave broadcast, bringing news, propaganda, and culture from one country to another. Pretty interesting to browse.
D). 6,000 Hz or 6 KHz. No. Not used for communication, for an interesting reason: This frequency is smack in the middle of the human hearing range. So if it were used for communication ... with high-power transmitters here and there ... then you wouldn't hear it in the air. But wherever wires were being used to carry sound ... your stereo's speaker wires, wires from your player to your ear-buds, wires to the telephones in your house etc ... the wires would act as antennas, picking up broadcasts at 6 KHz, and the broadcasts would get into everything. Not a smart plan.
On page 103 it explains that The radio frequencies used by commercial radio broadcasting stations range from about 550,000 Hz to 1,700,000 Hz. So the correct answer is A. 600,000 Hz
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