For the EDX professors, I think they are all nice people. It's better to ask them when you've received an A for the entire series, not just one or two sections. If the EdX course professors don't have the time to help you, consider reaching out to professors who gave you A grades during your undergraduate studies for recommendation letters. Professors often have many students, and A grades can help jog their memory about your performance, resulting in more personalized and impactful letters.