Why did the egyptians call their king pharaoh?

a.) the egyptians borrowed the title from the sumerians.
b.) the king wore the double crown of egypt, and the word means "he who rules all".
c.) thinking it was disrespectful to speak directly about a god-king, they referred to the palace instead.
d.) calling such a powerful ruler king was not enough, and the word pharaoh means god-king.