Find out whether there is any grammatical error in below sentence.
We discussed about the problem so thoroughly / on the eve of the examination / that I found it very easy to work it out.
A. We discussed about the problem so thoroughly
B. on the eve of the examination
C. that I found it very easy to work it out.
D. No error.
Answer: Option A

It was my common mistake!
Thank you
Because here word discuss as a verb and as verb it don't takes preposition ..But as a noun it takes.. Preposition
But why
Y it's error
Why??