Astronomers Originally Classified the Spectra of Stars According to the Strengths

Astronomers originally classified the spectra of stars according to the strengths of their hydrogen lines. What new physical insight enabled a reinterpretation of these patterns in terms of surface temperature during the 1920s?

A) discovery of nuclear physics and how stars generate their energy
B) discovery of blackbody radiation and the blackbody curve
C) development of atomic physics and how atoms emit light
D) development of radio astronomy and the detection of molecules in space