# Let $f:R\rightarrow R$ be a differentiable function and $f(1)=4$, then the value of $\large \int_4^{f(x)}\large\frac{2t\;dt}{x-1}$ is
$(a)\;8f'(1)\qquad(b)\;4f'(1)\qquad(c)\;2f'(1)\qquad(d)\;f'(1)$