A series of letter pairs is given with one pair missing. Choose the option that completes the pattern: CD, ?, MN, UV, EF.

Introduction / Context:
This problem involves letter pairs where the starting letters move through the alphabet with steadily increasing step sizes. You are asked to determine the second pair in the sequence. Because the letters wrap around the alphabet, modular arithmetic is required to check the pattern correctly.

Given Data / Assumptions:

• Sequence: CD, ?, MN, UV, EF. • Each pair consists of two consecutive letters (C with D, M with N, etc.). • Alphabet positions: A = 1 to Z = 26. • We assume a regular pattern of growing step sizes between starting letters.

Concept / Approach:
Because each pair is made of consecutive letters, we can focus on the first letters and treat them as a numeric sequence. We look for a pattern in the steps from the first letter of one pair to the first letter of the next. Then, we confirm that the same pattern can explain the entire sequence, including wrapping from Z back to A.

Step-by-Step Solution:
Step 1: Note the known first letters. CD starts at C (3), MN starts at M (13), UV starts at U (21), EF starts at E (5). Step 2: Examine steps between known first letters. From M (13) to U (21) is +8. From U (21) to E (5) is +10 modulo 26 (21 + 10 = 31, 31 − 26 = 5). This suggests a pattern of increasing step sizes. Step 3: Propose a sequence of steps: +4, +6, +8, +10 for the first letters. Starting at C (3), a +4 step gives 7, which is G. Adding +6 yields 13, which is M. Adding +8 yields 21, which is U. Adding +10 yields 31, which wraps to 5, E. Step 4: This sequence 3, 7, 13, 21, 5 matches C, G, M, U, E exactly and respects the +4, +6, +8, +10 pattern. Therefore, the missing first letter is G, and the pair must be GH since each pair contains consecutive letters.

Verification / Alternative check:
The full series of pairs becomes CD, GH, MN, UV, EF. Starting letters C, G, M, U, E now follow the increasing jump pattern, and second letters D, H, N, V, F naturally accompany them as next letters in each pair. No other option provides a first letter that produces this neat run of step sizes.

Why Other Options Are Wrong:
• EF: This is already present at the end of the sequence and cannot correctly serve as the second term. • KL: This would give a starting letter K, which does not fit the proposed +4, +6, +8, +10 pattern originating from C and ending at E. • ML: This starts with M, which is already used as the third pair start, and disrupts the increasing step structure.

Common Pitfalls:
Some candidates attempt to use a constant step size for the first letters, which fails here. Others may ignore the wrap around from U to E and misinterpret the last step. Carefully testing increasing step patterns and handling wrap around explicitly prevents these mistakes.

Final Answer:
The pair that correctly completes the series is GH.