Which letter should replace the question mark in the series C, X, F, U, I, R, ?

Two runs are interleaved. The terms in the 1st, 3rd and 5th places are C, F, I, climbing three at a time; the terms in the 2nd, 4th and 6th places are X, U, R, falling three at a time. The seventh term belongs to the climbing run, and three ahead of I (9th) is L (12th). O (15th) is the next term of the falling run and so would be the eighth term, not the seventh; J is only one ahead of I and K only two.