I'm running into a problem of reminding my professors of LORs. The letters are due on Dec. 1st and besides sending them reminders thru application systems, I've also sent them emails 4 weeks ago to remind them that the letters are due on the 1st of December. But my professors didn't respond to my emails at all, and as the deadline approaching, I'm really worried about how to write the reminder emails. I wanna make sure they understand it's urgent, and I wish they could have it written ASAP. But i also don't want to be rude.
How should I phrase this email?