Active to passive: modal perfect (could/might/must have + past participle -> modal have been + past participle)

1. A.
   Her research might be completed by her by that time.
2. B.
   Her research might had been completed by her by that time.
3. C.
   Her research might have been completed by her by that time.
4. D.
   Her research might have completed by her by that time.

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1. A.
   A reply could have been send by Meenakshi.
2. B.
   A reply could have be sent by Meenakshi.
3. C.
   A reply could have been sent by Meenakshi.
4. D.
   A reply could have sent by Meenakshi.

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1. A.
   The encryption keys must have been tampered with.
2. B.
   The encryption keys must have tampered by someone.
3. C.
   The encryption keys must be tampered with by someone.
4. D.
   The encryption keys must had been tampered with.

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1. A.
   The house could have been sold by the agent.
2. B.
   The house could be sold by the agent.
3. C.
   The house could have sold by the agent.
4. D.
   The house could had been sold by the agent.

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