Take another look at the item:

Mrs. Neal loves chocolate cake. (A) But couldn't accept the slice that her student Michael offered (B) because she knew that it was a bribe (C) for a better grade on his midterm essay.

You wanted to do this:

Mrs. Neal loves chocolate cake. But couldn't accept the slice that her student Michael offered because she knew that it was a bribe, for a better grade on his midterm essay.

A comma after bribe is an error. For a better grade on his midterm essay is a string of prepositional phrases. When you attach such phrases at the end of a sentence, you do not need punctuation.

You might want to review the rules.

Go back to the sentence to try again.

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