Mr.Joe,_____ professor at the University of Fribourg,is famous for his academic achievements.
A.不填 B.a
翻译成“***大学的一个教授”
但是我觉得,“professor at the Universit of Fribourg”是作Mr.Joe的同位语,当称谓、头衔作同位语时,要使用零冠词的.