Which of the following would be a subsequent discovery of facts which would require a response by the auditor?

Which of the following would be a subsequent discovery of facts which would require a response by the auditor?

a. Discovery of the inclusion of material nonexistent sales.
b. Discovery of the failure to write off material obsolete inventory.
c. Discovery of the omission of a material footnote.
d. Each of the above would require a response by the auditor.