Take another look at the item:

While Dr. Kinser bounced down the stairs (A) and strode through the hallway. (B) A brightly colored tie streamed over his shoulder, (C) flapping in his wake.

You wanted to do this:

While Dr. Kinser bounced down the stairs and strode through the hallway. A brightly colored tie streamed over his shoulder flapping in his wake.

Removing the comma after shoulder is an error. Flapping in his wake, a participle phrase, needs the comma to connect it to the main clause in front.

You might want to review the rules.

Go back to the sentence to try again.

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