Deduction under section 80gg, am i eligible for it ?

Dear CA club members,

Please help me with following queries related to deduction under section 80GG

1) I am out of job for last couple of years due to health issues, I have no income from salary/profession. I dont own a house/flat and I live in a rented flat. Can I claim deduction under 80GG ?? But I have income from interest earned from fixed deposits.

How much deduction can I claim ? 

2) If I am eligible for deduction under 80GG, can I revise my past returns for FY2015-16, FY2016-17 and claim for deductions under 80GG for previous financial years

Thank