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:
Intensive Course
A concentrated and rigorous course of study designed to cover large amounts of material in a short period of time.
Semantic Processing
The process of understanding and deriving meaning from words and sentences in language.
Long-Term Memories
Memories that can be retained for an extended period of time, ranging from days to decades.
Testing Effect
The enhanced memory retention and learning that occurs from retrieving information through testing, compared to simply rereading or restudying material.
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