How Can We Explain the Fact That Hydrogen Balmer Lines

Multiple Choice

How can we explain the fact that hydrogen Balmer lines are essentially absent in the hot O type and the cool M stars?

A) O type stars do not contain hydrogen.
B) In cool stars hydrogen combines to form large molecules.
C) In O stars hydrogen is ionized and in M stars the electrons of hydrogen are in level 1, not level 2.
D) All of these choices are correct.