What is the working substance in an external combustion engine and an internal combustion engine?

For the given $P-V$ diagram of a thermodynamic system,match the curves with their respective thermodynamic processes. ($P$ = Pressure and $V$ = Volume)

Curve	Process
$I$	$a)$ Adiabatic
$II$	$b)$ Isobaric
$III$	$c)$ Isochoric
$IV$	$d)$ Isothermal

$1$ mole of rigid diatomic gas performs a work of $Q/5$ when heat $Q$ is supplied to it. The molar heat capacity of the gas during this transformation is $\frac{xR}{8}.$ The value of $x$ is $\ldots \ldots \ldots .$ $[R =$ universal gas constant $]$

The three processes in a thermodynamic cycle shown in the figure are: Process $1 \rightarrow 2$ is isothermal; Process $2 \rightarrow 3$ is isochoric (volume remains constant); Process $3 \rightarrow 1$ is adiabatic. The total work done by the ideal gas in this cycle is $10 \, J$. The internal energy decreases by $20 \, J$ in the isochoric process. The work done by the gas in the adiabatic process is $-20 \, J$. The heat added to the system in the isothermal process is .............. $J$.

Check if the following statements are true or false:
$1.$ For an adiabatic process,$T V^{\gamma - 1} = \text{constant}$.
$2.$ The charging process of a battery is a reversible process.
$3.$ Water falling from a height is a reversible process.
$4.$ Internal energy,volume,and mass are intensive variables,while pressure,temperature,and density are extensive variables.

There are two samples $A$ and $B$ of a certain gas,which are initially at the same temperature and pressure. Both are compressed from volume $V$ to $\frac{V}{2}$. Sample $A$ is compressed isothermally while sample $B$ is compressed adiabatically. The final pressure of $A$ is