Again I would like to verify the solution from a workbook that I think may be incorrect. Here's the question:
Remove unnecessary commas in the sentence:
Tina remarked, "The bus has been late every day, since the new bus schedule came out, so I'm going walk instead."
Should be: "The bus has been late every day since the new bus schedule came out, so I'm going walk instead. "
Your first comma was wrong, and I'm not sure if I'm able to explain why, sorry!