Change the following from active to passive:
Someone must have altered the access codes.

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Accepted Answer

1. The sentence uses **modal + perfect**: "must have altered" (someone did it in the past).

2. In passive voice we need **"been" + past participle**. So it becomes: "must have been altered".

3. Check other options:
   - (b) misses **"been"** and is ungrammatical.
   - (c) uses the wrong tense (present instead of perfect).
   - (d) uses **"had"** wrong after must.

4. So option **(a)** is correct: "The access codes must have been altered."

FINAL_ANSWER: a

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