Secondary document for birth certificate verification

[vrathnam]

My wife is a US citizen applying for a GC for her parents. Looking at instructions here: https://www.uscis.gov/sites/default/...i-130instr.pdf

For birth certificate, her parents have the correct "non-availability of bc" document from India. As a secondary document, they also have secondary school leaving certificate which matches.

Do they need another supporting document in addition to all this, which would be an affidavit?

Thanks,