Given $x=4y$ and $y=2z$, which of the following expressions is equivalent to $x+4y-8z$ in terms of $z$ ?

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Write each variable in terms of $y$:

$$x=4y, y=2z$$ $$x=4(2z)$$ $$x= 8z$$

Substitute:

$$x+4y-8z$$ $$(8z)+4(2z)-8z$$ $4 = \boxed{8z} $$