A: Well, I can't exchange goods without a receipt.
B: Look -- this is the right box but the wrong CD. All I want is the right
CD.
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What is the function (role) of the dash(--) in the sentence?
The dash is just providing a slight break in thought and a pause in speech.
I'd punctuate that answer with an exclamation mark, though.
Look! This is the right box ...