David accidentally omitted from his 2016 tax return $40,000 of the gross receipts that he collected as the owner of a restaurant. His 2016 return indicated collective gross receipts of $25,000. The IRS no longer can pursue David with the threat of collection of the related tax, interest, and penalties, as of April 15 of what year?

a. 2018

b. 2021

c. 2023

d. 2024

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