Answer
The arc length doubles.
Work Step by Step
Arc length s (radius r, central angle $\theta$):$ \quad s=r\theta$, where $\theta$ is in radians
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Before the radius increases,
$ s_{1}=r\theta$.
After $r$ increases to $2r$ (and the central angle remains the same),
$s_{2}=(2r)\theta=2(r\theta)=2s_{1}$
The arc length doubles.
