Linn filed her 2020 Form 1040 on April 4, 2021. What is the date on which the applicable statute of limitations expires in each of the following independent situations?

a. Linn incurred a bad debt loss that she failed to claim.

b. Linn inadvertently omitted one-third of the correct gross income.

c. Same as part (b), except that the omission was deliberate.

d. Linn innocently overstated her deductions by a large amount.

e. No return was filed by Linn.