Here is the sentence:

Le’Ron scowled at Professor Nguyen, was muttering under his breath, and sighed heavily, but in truth he loved taking algebra exams.

When you list actions, you must use parallel structure. Here, scowled and sighed are both simple past tense verbs, but was muttering is past progressive, wrecking the parallelism. What a delectable choice you made!

To fix the sentence, change was muttering to muttered.

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