我只需要一支笔。

I only need ___one pen.

这是最后一个蛋糕了，你想要一个吗？

This is the last cake, do you want ___one?

这些杯子中，你最喜欢哪一个？

Which ___one of these cups do you like the most?

她是全班唯一一个考试满分的学生。

She's the ___only student in the class who got full marks on the exam.