One difficulty in using voluntary transactions to internalize externalities is that: _____________

a. the government usually will not enforce contracts of this type.
b. people are motivated by​ self-interest and are often unwilling to engage in a transaction that might make another person better off.
c. transaction costs of coming to an agreement can be very large when numerous people are involved.
d. people usually​ don't understand what the real opportunity costs are that they face.