Economic behavior is always rational

The above table shows the total product schedule for the campus book store. If each employee is paid $6 per hour and there are no other variable costs, then at what level of books sold per hour does the marginal cost begin to increase?

A) 41 books per hour B) 59 books per hour C) 73 books per hour D) 90 books per hour

The true causal effect might not be the same in the population studied and the population of interest because

A) of differences in characteristics of the population B) of geographical differences C) the study is out of date D) all of the above