= Solution
{c}
For $0<Q^2<M^2$ there are two distinct positive roots:
$$
\boxed{r_\pm=M\pm\sqrt{M^2-Q^2},\qquad F=\frac{(r-r_+)(r-r_-)}{r^2}.}
$$
Both roots are <coordinate singularities> of the static chart. In the standard extension, $r_+$ is the outer <event horizon> and $r_-$ is an inner <Cauchy horizon>. The <curvature singularity> is $r=0$. Between the horizons $F<0$, so the radial direction is timelike; for $0<r<r_-$, $F>0$ again and the central <curvature singularity> is timelike. These are the three radial regions of the <global structure of a charged spherical black hole>.<General Relativity by David Tong>[Sections 6.2.2–6.2.5, printed pages 260–268]{ref}
For completeness, if $Q=0$ and $M>0$, the <Schwarzschild spacetime> limit has just the positive horizon $r=2M$ and a spacelike <curvature singularity> at $r=0$; the formal root $r_-=0$ of $r^2F$ is not a regular inner horizon. If $M=Q=0$, the <metric tensor> is flat and the radial origin is regular in <Cartesian coordinates>.
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