$(A)$ Internal energies at $\mathrm{A}$ and $\mathrm{B}$ are the same
$(B)$ Work done by the gas in process $\mathrm{AB}$ is $\mathrm{P}_0 \mathrm{~V}_0 \ln 4$
$(C)$ Pressure at $C$ is $\frac{P_0}{4}$
$(D)$ Temperature at $\mathrm{C}$ is $\frac{\mathrm{T}_0}{4}$