I want to buy a computer but there is only one left in that shop so Ill have to buy .A.

问题补充：

I want to buy a computer, but there is only one left in that shop, so Ill have to buy .

A. one

B. it

C. that

D. them

答案：

A. one

(答案→)B. it

C. that

D. them

解析：因“there is only one left in that shop”，所选择的项是特指具体的物the only one，所以选B。