What is the opportunity cost?
It is the next best alternative forgone upon making a choice. The opportunity cost is the value of that which must be given upon to acquire or achieve another option.
Why the opportunity cost exist?
Because of scarcity. Due to unlimited wants, but limited resources, we are forced to make choices.
If resources were limitless, no action would be at the expense of any other - the opportunity cost of any action would be zero.
What diagram helps to explain opportunity cost?
The Production Possibility Curve (PPC).