Can I help you ?- I’d like to buy a present for my best friend at proper price but

问题补充：

--- Can I help you ?

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

A. that

B. one

C. it

D. this

答案：

A. that

(答案→)B. one

C. it

D. this

解析：one可以用来指代前面提到的人或事物，此句中指代a present，one作present的同位语。