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

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：

A. One

B. It

C. That

D. This

解析：

此处one泛指上文提到的某只“pen”。