At/In/On: He's is an expert at/in/on woman psychology

[Tung Quoc]

Dear teachers,

Please read:

1. He's is an expert at/in/on woman psychology.
2. He's is an expert at/in/on modern literature.
3. She's an expert at/in/on getting her own way.

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"?

Quoc

[Pamela]

Consult Oxford dictionary :D Prepositions are given.Everything is clear there