--- 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：此处用one表示泛指。keep looking继续找。