In a two letter alphabet series reasoning question, the terms HI, OP and WX are given. Which pair of letters should come before HI so that the overall series is completed correctly?

Introduction / Context:
This problem belongs to the alphabet series category in verbal reasoning, where each term is a pair of letters from the English alphabet. The series is written as ?, HI, OP, WX. Our task is to determine which two letter pair must appear at the beginning so that the complete sequence follows a consistent and logical pattern. Such questions test familiarity with alphabet positions and the ability to detect numeric jumps between letters when they are treated as ordered symbols rather than simple characters.

Given Data / Assumptions:

• Known pairs in order: ?, HI, OP, WX.• Each term consists of two consecutive letters: H I, O P and W X.• We treat the first letters as one sub series (H, O, W) and the second letters as a parallel sub series (I, P, X).• Alphabet positions are A=1, B=2, ..., Z=26.• The missing pair must preserve a smooth numeric pattern in both sub series.

Concept / Approach:
The standard approach is to focus on the first letters of each pair and then the second letters separately. If the positions form a regular pattern, we can extend that pattern backwards to find the missing term. Because the pairs HI, OP and WX already show a clear stepwise increase in letter positions, we expect that the unknown pair will fit into the same numeric rule when the alphabet is treated cyclically from A to Z.

Step-by-Step Solution:
Step 1: Convert first letters to numbers: H=8, O=15, W=23.Step 2: Compute the differences: 15 − 8 = 7, 23 − 15 = 8. So the first letters increase by +7 and then +8.Step 3: To find the term before H, we continue the pattern backwards with a difference of +6. So the unknown first letter should satisfy x + 6 = 8, giving x = 2 which corresponds to B.Step 4: Now consider the second letters: I=9, P=16, X=24.Step 5: Differences are 16 − 9 = 7 and 24 − 16 = 8, the same pattern of +7 and +8.Step 6: Going backwards with +6 again, the unknown second letter must satisfy y + 6 = 9, so y = 3 which corresponds to C.Step 7: Therefore, the missing pair at the beginning of the series is BC.

Verification / Alternative check:
Write the full series using the discovered rule for the first letters: B (2), H (8), O (15), W (23) with differences +6, +7, +8. Do the same for second letters: C (3), I (9), P (16), X (24) with the same differences. This symmetry in both positions confirms that the sequence is consistent and that BC is the only pair which can be placed before HI to keep the pattern intact.

Why Other Options Are Wrong:
AB would give first letter A=1, leading to a difference of 7 from A to H, which would break the intended +6, +7, +8 pattern. DE and EF similarly produce inconsistent gaps when compared to H and I. None of these alternatives allow us to construct a clean backward extension with a +6 step in both positions, so they cannot complete the series logically.

Common Pitfalls:
Many learners try to guess the answer by rough visual spacing or by assuming that every series always moves forward from the first term. Here the unknown term is at the beginning, so the pattern must be extended backward. Another common error is to check only the first letters while ignoring the second letters. A correct solution must satisfy the same numeric logic in both positions of each pair.

Final Answer:
BC