Correct Answer:
C. than
Explanation:
The correct answer is than because 'No sooner' is a comparative correlative adverbial phrase that strictly pairs with 'than'.
- Step 1 (Correlative Pair): In English grammar, the negative adverbial formula 'No sooner... than' is a fixed idiom used to showcase an action that happens immediately after another. Because 'sooner' is a comparative adjective, it naturally demands 'than'.
- Step 2 (Sentence Structure): The inversion 'had we reached' is completed by introducing the second rapid event with 'than'.
- Incorrect Options:
- then is a common error; while it indicates time progression, it is grammatically incorrect to use with the comparative 'sooner'.
- when is incorrect because 'when' is exclusively paired with phrases like 'Hardly had...' or 'Scarcely had...', not 'No sooner'.
- where is incorrect because it denotes location and has no place within this time-based correlative structure.