If $f$ is a differentiable function, then $f'(b)$ is given by which of the following?

I.$\lim \limits_{x \to b} \cfrac{f(b)-f(x)}{b-x}$

II.$\lim \limits_{h \to 0} \cfrac{f(b+h)-f(b)}{h}$

III.$\lim \limits_{h \to 0} \cfrac{f(b+h)-f(b-h)}{2h}$

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