A tank is divided into two compartments with one compartment containing a gas at a given pressure, while the second is completely evacuated. If the partition is removed, the gas occupies the entire compartment. Which one of the following statements is $\textsf{CORRECT}$?
- Work done is equal to the area under the curve of a $p-V$ diagram
- Expansion of gas is not restrained by external force
- The entire process is reversible
- The overall change in volume is zero for the entire system
