Read the excerpt from A Black Hole is NOT a Hole. It's terrific fun to read what the original thinkers had to say for themselves. I read parts of Relativity: The Special and General Theory by Albert Einstein, and found the letter about Einstein forgetting to eat lunch in Einstein: His Life and Universe by Walter Isaacson.

I also reviewed parts of Isaac Newton's Principia: The Mathematical Principles of Natural Philosophy (translated by Andrew Motte), to see what Newton himself had to say.

What makes these books credible sources?

A: The books give opinions and arguments about science.
B: Einstein and Newton are experts in their fields.
C: The books describe Einstein’s and Newton’s inner feelings.
D: Einstein and Newton persuade readers to their opinions.