The commercial value of ivory is a threat to the elephant, but the commercial value of beef is a guardian of the cow. This is because
A.
a. the cow is raised in developed countries, while the elephant lives primarily in less-developed countries.
B.
b. cows are private goods, while elephants tend to roam freely without owners.
C.
c. cows and elephants are public goods, but ivory is nonrival.
D.
d. ivory is nonrival and nonexclusive, but beef is rival and exclusive.