Although they feel tired they still go on working. A. andB. butC. /D. so

问题补充：

Although they feel tired,they still go on working.

A. and

B. but

C. /

D. so

答案：

A. and

B. but

(答案→)C. /

D. so

解析：根据although“尽管”可知，本句是一个让步状语从句。在英语中，如果有了although，就不能用but；用了but，就不能用although。本句的意思是“尽管他们感到很累，但是他们仍然继续工作。”所以本题选C。