Can two isothermal curves cut each other?

In an isothermal expansion,what happens to the internal energy of the gas?

An ideal gas at pressure $P$ is enclosed in a container that is placed in a reservoir at temperature $T$. If the volume of the gas is increased to two times its original value, then the new pressure $P^{\prime}$ is equal to:

$A$ sample of an ideal gas undergoes an isothermal expansion. If $dQ$,$dU$,and $dW$ represent the amount of heat supplied,the change in internal energy,and the work done respectively,then:

$A$ poly-atomic molecule $(C_v = 3R, C_p = 4R$,where $R$ is the gas constant) goes from phase space point $A (P_A = 10^5 \ Pa, V_A = 4 \times 10^{-6} \ m^3)$ to point $B (P_B = 5 \times 10^4 \ Pa, V_B = 6 \times 10^{-6} \ m^3)$ and then to point $C (P_C = 10^4 \ Pa, V_C = 8 \times 10^{-6} \ m^3)$. The path $A$ to $B$ is adiabatic and the path $B$ to $C$ is isothermal. The net heat absorbed per unit mole by the system is:

$A$ thermodynamic process in which the temperature $T$ of the system remains constant,although other variables $P$ and $V$ may change,is called: