The 1420 MHz (21 cm) radio line generated by neutral hydrogen atoms is a good choice for interstellar communications because

A) it is the easiest radio frequency to generate
B) it is the only radio frequency that is not blocked by interstellar matter
C) all intelligent civilizations should be aware of its universal significance and would probably also think of sending and receiving signals at this frequency
D) it is the only frequency to which radio telescopes can be tuned


C

Physics & Space Science

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