Question

An author's credentials should be checked by:

A.current biography
B.book jacket
C.librarian
D.all of the above

Answer 1

All of the above: D

This would be correct.

Answer 2

Author's credentials should be checked by : 
A. current biography

For example, if the author is writing a book about finance, it will be helpful if we can find in the biography that he had several experience working in financial industries

hope this helps