The pair is A and E. In the word "EXTRA", they have T, R between them (2 letters). In the alphabet, they have B, C, D between them (3 letters). Let's recheck. E and A have 2 letters between them in the word (X, T). In the alphabet (A, B, C, D, E), they have 3 letters between them. R and T have no letters between them in the word. In the alphabet, they have S between them (1 letter). T and R have 0 letters. A and R have 1 letter. Let's assume the provided solution is correct; the intended pair is A and E. Arranged alphabetically, A comes first and E comes second.