Appendix A - Review - A.1 Algebra Essentials - A.1 Assess Your Understanding - Page A11: 45

$d(C,D)=1$.

The coordinate of point $C$ is $=0$ The coordinate of point $D$ is $=1$ The distance between $C$ and $D$ is $d(C,D)=\left | 1-0 \right |$ Simplify. $d(C,D)=\left | 1 \right |$ $d(C,D)=1$ Hence, the distance between $C$ and $D$ is $1$.