Which Type of Radiation Has Been MOST Effective in Evaluating $\alpha$

Multiple Choice

Which type of radiation has been MOST effective in evaluating the spiral arm structure of the Milky Way Galaxy?

A) 21-cm radio emission from electron spin-flip transitions in cool hydrogen gas
B) Lyman- $\alpha$ ultraviolet emission from hot hydrogen gas
C) synchrotron radiation from electrons spiraling in magnetic fields in the spiral arms
D) neutrinos from exploding stars in the spiral arms, since they can easily penetrate dust and gas