Question

I’d like to buy a present for my father’s birthday, ______ at a proper price but of great use.

---Can I help you?<br>---I’d like to buy a present for my father’s birthday, ______ at a proper price but of great use.
为什么是one不是which？说明理由，谢谢！

Answer 1

which是就近修饰，在这里修饰birthday，表意错误有歧义

Answer 2

one与之前的a present是同位语关系，不存在修饰的关系。
记住：a + 名词, one + 定语
the + 名词, the one + 定语