He is very old he still works very hard.A. butB. ifC. whenD. as

问题补充：

He is very old, he still works very hard.

A. but

B. if

C. when

D. as

答案：

(答案→)A. but

B. if

C. when

D. as

解析：考查连词：A. but 但是，B. if 如果，C. when当…时候，D. as正如，因为，随着，句意：他老了，但是还是工作很努力。两句话是转折关系，选A。