Take another look at the item:

Once classes were over, (A) Maria used to meet her lab partner Chris at the diner near campus (B) until she discovered that Chris preferred, (C) to flirt with the waitresses instead of discussing their chemistry homework.

You wanted to do this:

Once classes were over, Maria used to meet her lab partner Chris at the diner near campus, until she discovered that Chris preferred, to flirt with the waitresses instead of discussing their chemistry homework.

You do not need a comma after campus. Maria used to meet her lab partner Chris at the diner near campus is the main clause. Until she discovered ... is a subordinate clause. Generally, you do not use punctuation to connect a subordinate clause to the end of a main clause.

You might want to review the rules.

Go back to the sentence to try again.

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