Question

Rational behavior requires thinking at the margin. which example represents this type of thinking?

Answer 1

The examples are:
-deciding whether a second burger is worth the extra $2 
-deciding whether the overtime pay is worth working on your day off
-deciding whether to pay a fine for polluting the local harbor or installing antipollution machinery

In rational behavior, our decision-making process is always based on which decision give the maximum/optimal benefit for use, after deducting all possibilities with a logical approach.
All the actions above represents this very concept of rational behavior.