I was disappointed with the film .I had expected ______ to be much better.为什么不能用one?理解为"我原以为是一部更好看的"?

I was disappointed with the film .I had expected ______ to be much better.为什么不能用one?理解为"我原以为是一部更好看的"?
I was disappointed with the film .I had expected ______ to be much better.
为什么不能用one?理解为"我原以为是一部更好看的"?

I was disappointed with the film .I had expected ______ to be much better.为什么不能用one?理解为"我原以为是一部更好看的"?
it 是指代词,指代前面的the film
one 一个,强调数量
e.g. This coat doesn't fit me well. Can you show me another one?
（与原来不同的）

it
翻译：我对这部电影很失望。我以为它比我预料中更好。
这里的it代替前面的film.如果是one的话就不是前面所说的film了，是其他任何一部电影。