You have found one of the equations, but there is another one. Remember that $\displaystyle{{b}}^{{y}}={x}\usepackage{cmbright}\Huge\$ is equivalent to $\displaystyle{\log}_{{{b}}}{x}={y}\usepackage{cmbright}\Huge\$ . When the exponent is 0, then $\displaystyle{y}={0}\usepackage{cmbright}\Huge\$ . The correct answer is I and III.