A pure sample of atomic gas has atoms with six principal quantum numbers that can yield several emission lines. What is the number of possible electron shell transitions for which energy is radiated? (Hint: if an electron has jumped form n = 1 to n = 6, it can have these transitions: n=6 to n=5, n=6 to n=4, n=6 to n=3, n=6 to n=2, n=6 to n=1. That's 5 of them – can you find the rest? Remember, the electron didn't have to jump all the way to the n=6 level to start out with.)

A. 9
B. 12
C. 14
D. 15