Which letter pair should replace the question mark in AB, CE, FI, JN, ?

OSCorrect answerYour answerNSCorrect answerYour answerOTCorrect answerYour answerPTCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer C

Correct answer: C

First letters: A, C, F, J (positions 1, 3, 6, 10), jumps 2, 3, 4. Next jump 5 gives O (15).
Inside each pair the gap grows: A to B is 1, C to E is 2, F to I is 3, J to N is 4.
So the second letter is 5 after O: $15 + 5 = 20$ , which is T. The pair is OT.

Worth noting

OS keeps the gap at 4. Two separate rules are at work (between pairs and inside a pair), and each must be continued.

Ten questions from Number and Letter Series, at your own pace.

Ten questions from Number and Letter Series, at your own pace.

Explanation