A closed system undergoes a process 1-2 for which the values of Q1-2 and W1-2 are +20 kJ and +50 kJ, respectively. If the system is returned to state 1 and Q2-1 is -10 kJ, What is the value of the work $W_{2-1}$?
(a) + 20 kJ (b) -40 kJ
(c) -80 kJ (d) + 40 kJ