What is true of both primary and secondary sources?

A. They both may be essays.

B. They are both very reliable.

C. They both may contain bias.

D. They are both first-hand accounts.

C. They both may contain bias.

It's a matter of perspective.

A primary source provides facts about an event, object, person, or work of art. Primary sources include facts on historical and legal documents, eyewitness accounts, results of experiments, statistical data, pieces of creative writing, audio and video recordings, speeches, and art objects.

So i'd say B. is the answer since they are both about real things.