Which of the following is the best example of making decisions at the margin?
a.John has been taking 15 units of college classes and then he decides to drop out of school to pursue professional surfing.
b.Marisa has signed up for 16 units of college classes and then she decides to drop a one-unit class.
c.Pekkei was not planning to attend college this semester, but then she decides to take 12 units worth of classes.
d.Jacob was taking 12 units of college classes and then he drops out of school to work at his father's bakery full-time.