[calidris]

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His check raise on the turn tells me that he has a 5

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or a K, or two 7's...
Point is that you can't be that sure he's got exactly a 5.

That said, I'd check the flop too. C-betting here is not for value and there's no reason to think a bet will fold some unwanted hands.

SB raises the turn and so has a 5, K, or 77. Since the K is statistically speaking less likely, I'd fold here too. And, as has been mentioned, we're not closing the action.