Final answer:
The sentence is missing necessary commas after introductory phrases. The correct sentence should be: 'On game days, after the crowd leaves, volunteers pick up the trash.' This provides clarity and separates the time frame from the main action.
Step-by-step explanation:
The sentence in question is 'on game days after the crowd leaves volunteers pick up the trash.' The issue with this sentence is not a comma splice or a prepositional phrase not set off, but rather a lack of punctuation that makes the sentence unclear. Specifically, the sentence is missing a comma after the introductory phrase.
An introductory phrase should be followed by a comma to separate it from the main clause of the sentence. The corrected sentence would be: 'On game days, after the crowd leaves, volunteers pick up the trash.'
The introductory phrases 'On game days' and 'after the crowd leaves' set the time frame for the action and should be separated from the main clause with commas for clarity.