The old man has useful horse and horse can do a lot of things for him.A. an; aB. the; a C.

问题补充：

The old man has useful horse and horse can do a lot of things for him.

A. an; a

B. the; a

C. an; the

D. a; the

答案：

A. an; a

B. the; a

C. an; the

(答案→)D. a; the

解析：这个老人有一匹有用的马，这匹马能为他做许多事情。结合语境可知前文泛指任何一匹有用的马，故用不定冠词。Useful的词首音素是辅音，故不定冠词用a。下文特指前文提到的那匹马，故用定冠词the，选D。