Complete the third conditional: “If I had known about the meeting, I ___ attended it.”

A little clarity goes a long way.

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  A (will have):

  “Will have” forms a future perfect construction; this sentence imagines a different past.
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  B (would have):

  A third conditional pairs “if + past perfect” with “would have + past participle”.
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  C (would):

  “Would” alone requires a base verb; “would attended” is ungrammatical.
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  D (had):

  “Had attended” is past perfect, not the conditional consequence required here.