Pick out the incorrect statement among the following 

1.  In a cyclic process the change in volume is zero but work done is not zero so (1) is incorrect statement.
2.  For a gas which obey’s  $P{V}^x=$constant
$C=\frac{R}{\gamma -1}+\frac{R}{1-x}$
We can see that x can have a range of values, which can make  C  negative. Hence it is correct.
3. $\text{d}U=n{C}_{\text{v}}\text{d}T$ , for an isothermal process, change in temperature is zero, hence there will be no change in internal energy.
4. Using first law of thermodynamics $\text{d}Q=\text{d}U+\text{d}W$ , for adiabatic process  $\text{d}Q=0$
Hence,  $\text{d}U=-\text{d}W$. Therefore, during expansion work done is positive, the internal energy decreases.
Therefore, the correct answer is  $\left(\text{A}\right)$