Let $f:\mathbb{R}\rightarrow \mathbb{R}$ be a continuously differentiable function such that $\left | f\left ( x \right ) -f\left ( y \right )\right |\geq \left | x-y \right |$, for all $x,y\in\mathbb{R}$. Then the equation ${f}'\left ( x \right )=\frac{1}{2}$
- has exactly one solution
- has no solution
- has a countably infinite number of solutions
- has uncountably many solutions