Should it not be your backs are wet then
The correct answers should be "your back is wet" or "Your backs are wet" It is incorrect English to say "Your Backs wet".
Incorrect. Please read the existing comments on this thread.
Kalian is plural, ie the speaker is speaking to more than one person, (and obviously a person cannot have more than one back) therefore "Your backs are wet" is the correct translation.
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Learning the sentences that matter