A line continues infinitely in both directions. This is indicated by arrows on both ends, so $\displaystyle{\stackrel{{\leftrightarrow}}{{{A}{C}}}}\usepackage{cmbright}\Huge\$ is the only line. A ray continues infinitely in one direction. This is indicated by an arrow on the end. For each of the four arrows in the picture there is a ray: $\displaystyle{\stackrel{{\to}}{{{B}{D}}}}\usepackage{cmbright}\Huge\$ , $\displaystyle{\stackrel{{\to}}{{{C}{A}}}}\usepackage{cmbright}\Huge\$ (points to the left), $\displaystyle{\stackrel{{\to}}{{{A}{C}}}}\usepackage{cmbright}\Huge\$ (points to the right), and $\displaystyle{\stackrel{{\to}}{{{A}{B}}}}.\usepackage{cmbright}\Huge\$