✔ 最佳答案
it's the difference between a singular (i.e. 'one') and a plural (i.e. 'ones'). use one/ones to avoid repetition避免重複. compare:
(1) I need a camera. Can you lend me ONE (=THE CAMERA)?
(2) The men in the screen are THE ONES (=THE MEN) I saw last night.