Do gift cards cover taxes?

Sorta unrelated, but I recently redeemed some gift cards and I'm wondering if they cover taxes. Lets assume I buy something that is $50 with a gift card of $55, and there is a tax of $5. So, does a) the card pays all $55 including taxes, leaving the card with a balance of $0, or b) does the card pay only $50 without tax, and there is a $5 balance remaining on the card, and now I need another payment method to handle the remaining $5.

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