Sit down.
AnonymousSit down.

This is a command, which cannot be made passive. Besides, a sentence needs a direct object in order to be made passive.
AnonymousSit down.
Be seated.

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Please don't go there

The crowd laughed at the clown

This sentence cannot be changed into passive voice as after the verb 'what' is asked which is used to check whether the sentence can be changed into passive voice or not.But after the verb 'where' gives the answer of the question 'sit where--down' and not 'sit what'.

Other reason is, that the sentence itself shows command or order so it simply means that the sentence is Intransitive and cannot be changed into passive voice.

Sit down please... Change the voice

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