Active to passive: modal perfect continuous (could/may have been + -ing -> have been being + past participle)

1. A.
   Differential topology could have be being explained for hours by the professor.
2. B.
   Differential topology could have explained for hours by the professor.
3. C.
   Differential topology could have been being explained for hours by the professor.
4. D.
   Differential topology could have had been been explained for hours by the professor.

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1. A.
   The intellectual property portfolio may have be being undervalued.
2. B.
   The intellectual property portfolio may have undervalued.
3. C.
   The intellectual property portfolio may have been being undervalued.
4. D.
   The intellectual property portfolio may have had been been undervalued.

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