Preposition At/in/on: They're all expert at/in/on this field

Dear teachers,

Please read:

  1. They’re all expert at/in/on this field.
  2. He’s expert at/in/on making cheap.

Do you choose “at”, “in” or “on” for each sentence? For each sentence, all are possible and interchangeable? If not, how to distinguish when using “at”, when using “in” , when using “on”?


One short, simplistic answer:

At a skill
In a field
On a topic

Thanks Sir,

But what does “one short” mean? It’s an expression?


Hi Quoc,

Mr. Micawber gave you a short and simplistic answer.[YSaerTTEW443543]

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