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
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Indicate whether the statement is true or false
A(n) ____________________ eclipse occurs when the Moon is at its greatest distance from Earth and the Moon is new and at a node in its orbit
Fill in the blank(s) with correct word
How can we determine the reflectivity of an asteroid?
A) by comparing its brightness in visible light to its brightness in infrared light B) by taking a photograph of it C) by measuring its mass and radius D) by where it is located in the asteroid belt
In which eon of Earth's history did life originate?
A) Proterozoic B) Phanerozoic C) Archean D) Hadean