which keyring is used to search for a public keys?

By default, when verifying a signed file or a file with a detached signature, which keyring is used to search for a public keys?

A.
~/.gnupg/trustdb.gpg

B.
~/.gnupg/secring.gpg

C.
~/.gnupg/trustedkeys.gpg

D.
~/.gnupg/pubring.gpg