Consider an economy producing only two goods: wheat and machinery. Which of the following statements is/are true after considering the PPF curve?

A. If the economy is at a point outside the PPC, it indicates unemployment of resources.

B. Moving from one point on the PPC to another involves opportunity cost, because producing more of one good means producing less of the other.

C. An inward shift of the PPC may result from a natural disaster destroying resources, while outward shift may come from technological progress. 

D. If the PPC is concave to the origin, the marginal opportunity cost of producing a good increases as more of it is produced.