Change the following from active to passive: The professor could have been explaining differential topology for hours.

Differential topology could have be being explained for hours by the professor.
Differential topology could have explained for hours by the professor.
Differential topology could have been being explained for hours by the professor.
Differential topology could have had been been explained for hours by the professor.

Correct answer: Differential topology could have been being explained for hours by the professor.

Solution

The active sentence is in modal perfect continuous form: "could have been explaining." In passive voice, the object "differential topology" becomes the subject, and the verb phrase changes to "could have been being explained."

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