Read the sentence below to find out whether there is any grammatical error in it. The error, if any, will be in one part of the sentence. Choose the option with that part as your answer. If there is no error, choose 'D' as your answer. (Ignore the errors of punctuation, if any).

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.

Correct answer is A

We discussed the problem so thoroughly