A certain amount of an ideal gas is initially at a pressure $p_1$ and temperature $T_1$. First, it undergoes a constant pressure process $1$-$2$ such that $T_2$ = $3T_1$/$4$. Then, it undergoes a constant volume process $2$-$3$ such that $T_3$ = $T_1$/$2$. The ratio of the final volume to the initial volume of the ideal gas is

- $0.25$
- $0.75$
- $1.0$
- $1.5$