The correct answer is: B. Rising opportunity cost between two goods.
The opportunity cost of producing one good is the amount of another good that must be given up in order to produce one more unit of the first good. The opportunity cost of producing a good is not constant. It usually increases as more of the good is produced. This is because as more of a good is produced, resources that are used to produce that good become scarcer. This means that more and more of other goods must be given up in order to produce one more unit of the first good.
The shape of the production possibility curve (PPC) is concave because of the law of increasing opportunity cost. The law of increasing opportunity cost states that as more of a good is produced, the opportunity cost of producing that good increases. This is because as more of a good is produced, resources that are used to produce that good become scarcer. This means that more and more of other goods must be given up in order to produce one more unit of the first good.
The PPC is a graph that shows the maximum combinations of two goods that can be produced with a given set of resources. The PPC is concave because of the law of increasing opportunity cost. The law of increasing opportunity cost states that as more of a good is produced, the opportunity cost of producing that good increases. This is because as more of a good is produced, resources that are used to produce that good become scarcer. This means that more and more of other goods must be given up in order to produce one more unit of the first good.
The PPC is a useful tool for understanding the concept of opportunity cost. It can also be used to analyze the effects of changes in resources, technology, and preferences on the production of goods and services.