--There must be a dozen pens in this house but I can never find one when I need one. -

问题补充：

----- There must be a dozen pens in this house but I can never find one when I need one. ---- Keep looking. is sure to turn up.

A. One

B. It

C. That

D. This

答案：

(答案→)A. One

B. It

C. That

D. This

解析：此处One特指上文提到的“the one I need”。