--What can I do for you?-- I’d like to buy a present for my son at a proper price but of

问题补充：

--What can I do for you?

-- I’d like to buy a present for my son, at a proper price but of great use.

A. one

B. the one

C. which

D. that

答案：

(答案→)A. one

B. the one

C. which

D. that

解析：考查代词：分析句子结构：后半句 “at a proper price but of great use” 没有动词，不是一个完整的句子，所以不选连接词which 和 it，而one可以做代词，指代a present，the one有特指的意思，这里还没有选好礼物，不用特指。选C。