If you see that all the submit buttons are grey/gray in an MIT xPRO course, it is likely because the assessment submissions are no longer open. Once the course ends, the ability to submit answers is typically closed.
If you missed submitting answers to certain questions and would like to see the correct answers, you should look for the "Show Answer" button. This button allows you to view the correct answers to the questions that you missed. Please note that this button is usually available even after the course has ended.
If you have any further questions or concerns regarding this issue or any other aspect of the course, please don't hesitate to reach out to our support team. You can contact us by email at support@xpro.mit.edu, and we will be happy to assist you.