Astronomers Use Millimeter Wavelength Radiation to Find Giant Molecular Clouds

Astronomers use millimeter wavelength radiation to find giant molecular clouds. What is the source of this millimeter radiation?

A) vibrations of the hydrogen atoms in the H₂ molecule, the most abundant molecule in giant molecular clouds
B) emissions from the hot O and B stars newly formed in giant molecular clouds
C) shock waves formed when solar winds from newly formed stars collide with the gas in giant molecular clouds
D) rotational energy changes in the CO molecule