The phrase "By the time" indicates that one action was already in progress before another action in the past. Therefore, the Past Perfect Continuous Tense (had been waiting) is the correct choice: "By the time the train arrived, we had been waiting at the station."
By the time the train arrived, we ___ waiting at the station.
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Question context
This question is part of 🗣️ Verbal Ability within the broader 🧠 Aptitude Mastery subject and appears in the ASF Airports Security Force preparation collection. Reviewing the answer together with nearby questions helps reinforce the underlying concept and identify how it may appear in competitive exams.