In the sentence, "classroom" is the compound noun. It is formed by combining the two words "class" and "room" to create a single word that acts as a noun.
I checked all primary classrooms, no one was there. (Which word is a compound noun?
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Question context
This question is part of 🗣️ Verbal Ability within the broader 🧠 Aptitude Mastery subject and appears in the ETEA Educational Testing and Evaluation Agency preparation collection. Reviewing the answer together with nearby questions helps reinforce the underlying concept and identify how it may appear in competitive exams.