The So-Called Water Hole, a Region of the Radio Spectrum

The so-called water hole, a region of the radio spectrum chosen for searches for signals from intelligent life because galactic and Earth-based noise and atmospheric absorption are at a minimum, is so named because

A) water vapor (H₂O) has an intense laserlike emission line at this wavelength that extraterrestrials might use to communicate with Earth.
B) two astronomically important wavelengths, the 21-cm line of H and a line from the hydroxyl radical OH, are in this region, the letters H and OH signifying water.
C) water vapor emissions from planets at this wavelength will be a good indicator of life on other planets because water is essential for life as known.
D) water vapor absorption in Earth's atmosphere reaches a sharp minimum at this wavelength.