Take another look at the item:

To make his long shift at Bernie's Burger Emporium tolerable, (A) Kelvin used ketchup, (B) to make smiley faces on the burgers (C) that he assembled at the grill.

You wanted to do this:

To make his long shift at Bernie's Burger Emporium tolerable, Kelvin used ketchup, to make smiley faces on the burgers, that he assembled at the grill.

Putting a comma after burgers is a mistake. That he assembled at the grill is a subordinate clause. When you attach this type of clause at the end of a sentence, you usually do not need any punctuation.

You might want to review the rules.

Go back to the sentence to try again.

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