【正确答案】
范文:
Dear Sir,
Please excuse my absence from class today because I got a bad cold yesterday evening and failed to fall asleep well in the night. I felt even worse this morning. Therefore, I had to go to the doctor to have myself thoroughly examined. Thus, I am writing to ask you for a day's sick leave and assure that I'll resume my study if I feel any better tomorrow.
Sincerely,
Xiao Ming