Why does a gas get heated on compression?

Given below are two statements. One is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$ : If $dQ$ and $dW$ represent the heat supplied to the system and the work done on the system respectively,then according to the first law of thermodynamics $dQ = dU - dW$.
Reason $R$ : The first law of thermodynamics is based on the law of conservation of energy.
In the light of the above statements,choose the correct answer from the options given below:

If the heat given to a system is $35 \ J$ and the work done on the system is $15 \ J$,then the change in the internal energy of the system is ... $J$?

$A$ diatomic gas has an initial internal energy of $80 cal$. $A$ work of $18 cal$ is done on the gas and the gas releases heat energy of $42 J$. The final internal energy of the gas is

In a thermodynamic process,the pressure of a fixed mass of a gas is changed in such a manner that the gas releases $30 \ J$ of heat and $10 \ J$ of work is done on the gas. If the initial internal energy of the gas was $30 \ J$,then the final internal energy will be ........ $J$.

The first law of thermodynamics is concerned with the conservation of