Answer this question on the basis of the following information for a public good. Pa and Pb represent the prices that citizens (a) and (b) , the only two people in this nation, are willing to pay for additional units of a quantity (Qc) of the public good. Qs represents the quantity of the public good supplied by government at each of the collective prices. 
The collective willingness of this nation to pay for the fourth unit of the public good is
Definitions:
Behavior Therapy
A therapeutic approach focused on changing negative behaviors into positive ones through reinforcement and conditioning techniques.
Undesired Behavior
Behavior that is considered unacceptable or inappropriate by society, an individual, or a specific group.
Token-economy System
A behavioral modification technique based on systematic reinforcement, using tokens as conditional reinforcement that can be exchanged for desired rewards.
Partial Reinforcement
A conditioning schedule in which a reward is not provided every time the desired behavior is performed, leading to more resistant learned behaviors.
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