The 1420 MHz (21 Cm) Radio Line Generated by Neutral

question 18

Multiple Choice

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

---

Definitions:

Number Series

A sequence of numbers in which there is a specific relationship or pattern between successive numbers.

Fractions

Portions of a whole number, symbolized by two integers separated by a slash, with one part representing a portion of the whole.

Whole Numbers

Numbers without fractions or decimals; positive integers including zero.

Lowest Terms

The most reduced form of a fraction, where the numerator and denominator are at their smallest whole number ratio.