Radio Waves with 21-Cm Wavelengths Are Important Because They

Radio waves with 21-cm wavelengths are important because they

A) allow us to study the deep interiors of stars.
B) allow us to image magnetic fields directly.
C) allow us to study neutral hydrogen in the interstellar medium.
D) is produced by every object in the universe.
E) is the longest wavelength of light that can be naturally produced.