A

$$ \left(\frac {\upsilon_2}{\upsilon_1}\right)^{\gamma + 1} $$

B

$$ \left(\frac {\upsilon_2}{\upsilon_1}\right)^\frac {\gamma - 1}{\gamma} $$

C

$$ \left(\frac {\rho_1}{\rho_2}\right)^\frac {\gamma - 1}{\gamma} $$

D

$$ \left(\frac {\rho_1}{\rho_2}\right)^{\gamma - 1} $$