Which distance formula or formulas show(s) a direct variation?

I $\displaystyle{d}={50}{t}\usepackage{cmbright}\Huge\$

II $\displaystyle{d}={16}{{t}}^{{2}}\usepackage{cmbright}\Huge\$

III $\displaystyle{d}={r}\cdot{t}\usepackage{cmbright}\Huge\$