\[(a)\;2 \bigg(\frac{P}{b}\bigg)^{1/3} \quad (b)\;\bigg(\frac{P}{b}\bigg)^{1/3} \quad (c)\;\bigg(\frac{P}{b}\bigg)^{2/3} \quad (d)\;2 \bigg(\frac{P}{b}\bigg)^{2/3}\]