A cold hydrogen gas cloud in front of a distant quasar produces a dark absorption line at 486 nm (the Hβ line). After being excited by a passing shock wave, the same cloud emits a strong line at 656 nm (Hα) but shows almost no emission at 486 nm. Which statement best explains the presence of the 486 nm absorption line without a corresponding strong 486 nm emission line?
(a) The shock wave ionizes nearly all hydrogen atoms, preventing any emission at 486 nm, whereas 656 nm photons are produced when the ions later recombine.
(b) After the shock wave heats the gas, thermal motions Doppler-shift the 486 nm photons out of the observed band, so the emitted line is effectively absent, while 656 nm photons remain within the detected range.
(c) During absorption, electrons are promoted from n=2 to n=4, but after excitation most electrons de-excite first to n=3 and then to n=2, so 656 nm photons are produced while direct n=4→2 (486 nm) emission is rare.
(d) Emission at 486 nm is forbidden by quantum-mechanical selection rules, although absorption at that wavelength can still occur.