3 moles of an ideal monatomic gas performs $ABCDA$ cyclic process as shown in figure below. The gas temperatures are $T_A=400\;K,T_B=800\;K,T_C=2400\;K,$ and $T_D=1200\;K$. the work done by the gas is (approximately) $(R= 8.314\;J/mole K)$:

$(1) \;10\;kJ$

$(2) \;20\;kJ$

$(3) \;40\;kJ$

$(4)\;100\;kJ$

(2) 20 Kj