$O_2$ is collected over water at $20^{\large\circ}C$. The pressure inside shown by the gas is 740 mm of Hg. What is the pressure due to $O_2$ alone if V.P of $H_2O$ is 18 mm at $20^{\large\circ}C$

Question

$(a)\;722mm\qquad(b)\;240mm\qquad(c)\;720mm\qquad(d)\;480mm$