Which pair should replace the question mark in the series CE, FH, IK, LN, ?

The first letters run C, F, I, L — each three ahead of the last — so the next is O (15th). The second letters run E, H, K, N in the same way, so the next is Q (17th). The pair is OQ, and within each pair the second letter stands two ahead of the first, which OQ also satisfies. OP keeps the gap of two only in the earlier pairs and closes it to one; PR moves the first letter four ahead instead of three; NP moves it only two ahead.