Fill in the blank with the most appropriate option:
Only when the panel had concluded its year-long investigation _____ the depth of regulatory oversight failures become apparent.
Show answer & explanation
Correct answer: B
Explanation
Correct Answer: did
Explanation
• When a sentence begins with a restrictive or negative expression like 'Only when...', the main clause requires inversion (auxiliary verb + subject + main verb).
• The investigation was concluded first (expressed in the past perfect: had concluded), and the failures subsequently became apparent (simple past tense).
• To invert a simple past tense clause with the base verb become, the auxiliary did is used (did + subject + base form of the verb).
Quick Tip: Remember the inversion structure: Only when / Only after + clause + did + subject + V1 (base verb).