Where can investors, journalists, and other market stakeholders find out who has been signing a particular U.S. listed issuer's audit opinions as engagement partner? a. By looking up the appropriate Form AP filed with the PCAOB on the regulator's website for any particular audit year since 2016. b. By looking at the signature at the bottom of the audit opinion in the issuer's 10-K. c. By submitting a Freedom of Information Act request to the PCAOB. d. By seeing who shows up and takes questions at the issuer's annual meeting