Tangent cones and $C^1$ regularity of definable sets

Let $X\subset {\mathbb{R}}^n$ be a connected locally closed definable set in an o-minimal structure. We prove that the following three statements are equivalent: (i) $X$ is a $C^1$ manifold, (ii) the tangent cone and the paratangent cone of $X$ coincide at every point in $X$, (iii) for every $x \in X$,thetangentconeof$X$atthepoint$x$isa$k$-dimensional linear subspace of ${\mathbb{R}}^n$ ($k$ does not depend on $x$) varies continuously in $x$, and the density $\theta (X,x)<3\mathrm{/}2$.