When using combination D, Alex is producing one picture frame and 19 cutting boards per day. Producing a second picture frame per day would require him to move to combination C, reducing his production of cutting boards to 16 per day. Since this change involves producing 3 fewer cutting boards per day (19−16=3), the opportunity cost of producing the second picture frame per day is 3 cutting boards per day.
Similarly, using combination C, Alex is producing two picture frames and 16 cutting boards per day. Producing a third picture frame per day would require him to move to combination B, reducing his production of cutting boards to 11 per day. Since this change involves producing 5 fewer cutting boards per day (16−11=5), the opportunity cost of producing the third picture frame per day is 5 cutting boards per day.
Alex's opportunity cost of producing the second picture frame per day is 3 cutting boards per day while the opportunity cost of producing the third picture frame per day is 5 cutting boards per day. Hence, as Alex increases his production of picture frames, his opportunity cost of producing more picture frames increases. This change is an example of the law of increasing opportunity costs.