When posting in a chat room or message board, you should be ______.
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Brief
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Understandable
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Knowledgeable
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All of the above
Effective online communication requires being brief to respect others' time, understandable so your message is clear, and knowledgeable when relevant to the discussion. All three qualities are important for good online etiquette. Option D correctly combines these elements. Each individual quality alone isn't sufficient.
Good online communication in a chat room or message board benefits from being brief, understandable, and knowledgeable all at once — these aren't mutually exclusive traits but complementary aspects of clear posting. Since all three individual qualities are desirable, "All of the above" is the most complete answer.