Michal accidentally omitted from her 2016 tax return $4,000 of gross receipts she collected as the owner of a restaurant. Her 2016 return indicated collective gross receipts of $35,000. The IRS no longer can pursue Michal with the threat of collection of the related tax, interest, and penalties as of April 15 of what year?

a. 2018

b. 2020

c. 2023

d. 2024

e. There is no expiration date for the statute of limitations in this context.