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A) in panic

B) in a panic

I don’t know how to differentiate them. Are they interchangeable?

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teacherJapanI don’t know how to differentiate them. Are they interchangeable?

Here is a tool for you to use for all such questions. Study the results of curated usage "in the wild"

B. 425 results:

https://fraze.it/n_search.jsp?hardm=1&t=0&l=0&p=1&q=%22in+a+panic%22


A. 343 results:

https://fraze.it/n_search.jsp?q=%22in+panic%22&l=0&t=0&ffo=false&findid=-1&ff=

Comments  
teacherJapanI don’t know how to differentiate them. Are they interchangeable?

It's tricky. You might flee in panic, meaning you flee because you are panic-stricken.

But you might flee in a panic, meaning you flee for an unstated reason but are in a state of panic while doing so.

Students: We have free audio pronunciation exercises.
 AlpheccaStars's reply was promoted to an answer.
Oh..., it’s confusing!!! Thank you both for your help:-)