An ideal gas at pressure \( P \) and temperature \( T \) is expanding such that \( P T^3 = \text{constant} \). The coefficient of volume expansion of the gas is ____________ .
\( \dfrac{2}{T} \)
\( \dfrac{1}{T} \)
\( \dfrac{4}{T} \)
\( \dfrac{3}{T} \)