Correct statements from the following are
A. Potassium dichromate is an oxidising agent and it oxidises \(FeSO_4\) to \(Fe_2(SO_4)_3\) in acidic medium.
B. Sodium dichromate can be used as primary standard in volumetric estimation.
C. \(CrO_4^{2-}\) and \(Cr_2O_7^{2-}\) are interconvertible in aqueous solution by varying the \(pH\) of the solution.
D. \(Cr\)-\(O\)-\(Cr\) bond angle in \(Cr_2O_7^{2-}\) is \(126^\circ\).

Choose the correct answer from the options given below:

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.