Choose the missing terms out of the given alternatives.
Y, B, T, G, O, ?

The given sequence is a combination of two series :
I. Y,T, O and II. B, G, ?
I consists of 2nd, 7th and 12th letters from the end of the English alphabet, while
II consists of 2nd, 7th and 12th letters from the beginning of the English alphabet.
So, the missing letter in II is the 12th letter from the beginning of the English alphabet, which is L.