If $x, y, z \neq 0$, $a^x = b^y = c^z$ and $b^2 = ac$, which is correct?

• a$\dfrac{1}{x} + \dfrac{1}{y} = \dfrac{2}{z}$
• b$\dfrac{1}{x} + \dfrac{1}{z} = \dfrac{2}{y}$
• c$\dfrac{1}{y} + \dfrac{1}{z} = \dfrac{2}{x}$
• d$\dfrac{1}{x} + \dfrac{1}{y} = \dfrac{1}{z}$