Alexander Selkirk lives on Juan Fernandez Island. He spends his days gathering coconuts and spiny lobsters. If he spends all day gathering coconuts, he can collect 40 coconuts. If he spends all day collecting spiny lobsters, he can capture 20 in a day. His production possibilities curve is shown in the figure.

As Selkirk collects more coconuts, the opportunity cost of a coconut is ________.

A.

increasing

B.

constant

C.

unpredictable

D.

decreasing