Correct. In the diagram, $\displaystyle{\overline{{{C}{D}}}}\usepackage{cmbright}\Huge\$ is a diameter of the circle and $\displaystyle{\overline{{{A}{B}}}}\usepackage{cmbright}\Huge\$ is a radius. The radius is half the diameter. Because the length of $\displaystyle{\overline{{{C}{D}}}}\usepackage{cmbright}\Huge\$ is 24 millimeters, the length of $\displaystyle{\overline{{{A}{B}}}}\usepackage{cmbright}\Huge\$ is $\displaystyle\frac{{1}}{{2}}{\left({24}\right)}={12}\usepackage{cmbright}\Huge\$ millimeters.