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：句意:那位老人有一匹有用的马，马能够为他做一些事情。第一空表示泛指，强调是一匹，所以用不定冠词an;第二空还是指前面提到的那匹马，所以是特指，根据句意故选D。