Complete the letter analogy: CE : FI :: JL : ?

The first letter moves forward three places while the second moves forward four: C + 3 = F and E + 4 = I. Applying the same two steps to JL gives J + 3 = M and L + 4 = P, so MP. MO comes from adding 3 to both letters, NP from adding 4 to both, and NQ from pushing each step up by one (J + 4 = N, L + 5 = Q). Only MP keeps the two unequal steps in their right places.