Figure 2.2 -Figure 2.2 Presents a Production Possibilities Curve for a Country

Figure 2.2
-Figure 2.2 presents a production possibilities curve for a country that can either produce highways or provide people with medical care in a given year. The reason why the production possibilities curve is shaped as it is (bowed outward) is because inputs for healthcare and highways are:

A) perfectly substitutable.
B) substitutable, but not perfectly substitutable.
C) not substitutable at all.
D) used in precisely the same ratios.