Whose pen is this? – it's mine.

这是谁的钢笔？——是我的钢笔。

Whose pen is this? --- It's Kate's.

这是谁的笔？ 是凯特的。

Whose pen is this? Let me see. Oh, yes, it's Mary's.

这是谁的钢笔？我来看一看，哦，是玛丽的。