If We Are Going to "Listen" for Electromagnetic Radiation at 21

Multiple Choice

If we are going to "listen" for electromagnetic radiation at 21 cm from extraterrestrial sources,then we had better not generate too many waves at this wavelength from terrestrial sources.The standard FM broadcast band (as you can see by looking at your radio dial) spans frequencies from 88 MHz (million cycles per second) to 108 MHz.How does the frequency of the 21-cm signal compare with the FM band? The frequency of the 21-cm line is:

A) below the low frequency end of the FM band by a factor of about 9.
B) right in the middle of the FM band.
C) right at the upper end of the FM band.
D) higher than the top end of the FM band by a factor of about 13.