T, a calendar year taxpayer, filed her 2020 Federal income tax return on January 29, 2021, and received a tax refund check for overpaid 2020 taxes on May 17, 2021.

a) Assuming that T did not file a false return or have a substantial omission of income, what is the last date on which the IRS may assess an additional income tax liability against her for the 2020 tax year? Answer in Month/Date/Year

b) If T unintentionally had a substantial omission of income from her 2020 return, what is the last day on which the IRS may assess her an additional 2020 income tax liability? Answer in Month/Date/Year

c) If T had never bothered to file her 2020 tax return, would the IRS be able to assess her on additional 2020 income tax liability? Yes/No