Although he is very old he works very hard.A. andB. butC. /D. so

问题补充：

Although he is very old, he works very hard.

A. and

B. but

C. /

D. so

答案：

A. and

B. but

(答案→)C. /

D. so

解析：句意：尽管他很年长，但是他工作很努力。在英语句子中although和though绝对不能和but出现在同一个句子中，但是可以和yet连用；故选C。