In the pair series RS, WX, ?, GH, which pair of letters should replace the question mark to complete the pattern?

Introduction / Context:
You are given the series RS, WX, ?, GH and asked to find the missing pair of letters. The pairs all consist of two consecutive letters, and the first letters themselves form an arithmetic progression when viewed as alphabet positions. Such questions check your understanding of forward and wrap around movement in the alphabet.

Given Data / Assumptions:

• Pairs: RS, WX, ?, GH.
• Alphabet positions A 1 to Z 26.
• Within each pair, the second letter is exactly one step after the first.

Concept / Approach:
First, confirm the internal structure of each pair. RS has R followed by S, and WX has W followed by X, so we can assume the missing pair also contains two consecutive letters. Then, focus on how the first letters of the pairs R, W, ?, G progress numerically, allowing wrap around from Z to A. Once the step is identified, we can determine the missing first letter and then the second letter.

Step-by-Step Solution:
Step 1: Check internal pair pattern. RS has letters R 18 and S 19, which are consecutive. WX has W 23 and X 24, also consecutive. GH has G 7 and H 8, consecutive as well. Step 2: Therefore the missing pair must also be two consecutive letters, say X and Y, with Y directly after X. Step 3: Now look at first letters: R, W, ?, G. Step 4: Convert to positions: R 18, W 23, G 7. The step from 18 to 23 is plus 5. To go from the unknown first letter to G with the same step, we require a plus 5 move modulo 26. Step 5: Let the unknown first letter have position k. Then k plus 5 modulo 26 must equal 7. So k equals 2, which is B. Step 6: Confirm the sequence: R 18 plus 5 equals 23 W, and 23 plus 5 equals 28, minus 26 equals 2 B, and 2 plus 5 equals 7 G. Step 7: Since each pair uses consecutive letters, the second letter of the missing pair is C (position 3). Step 8: Therefore the missing pair is BC.

Verification / Alternative check:
The full series is RS, WX, BC, GH. Numerically this gives first letters 18, 23, 2, 7 with a constant step of plus 5 modulo 26, and second letters 19, 24, 3, 8 with the same step. This confirms that BC is consistent with the pattern for both letters.

Why Other Options Are Wrong:
AB and CD are consecutive pairs but do not place B as the first letter of the missing pair. EG is consecutive but it would disrupt the step sequence of first letters. YZ has letters near the end of the alphabet and does not fit the plus 5 pattern between W and G.

Common Pitfalls:
A common mistake is to look only at the visual similarity of pairs and choose any consecutive pair without checking the inter pair progression. Another is to forget about wrap around arithmetic when first letters pass Z and restart from A. Carefully checking numeric differences avoids such errors.

Final Answer:
The pair of letters that completes the series is BC.