Until January 1, 2012, the price for ethanol consumers in the United States was higher than world free-market price by $0.54 per gallon because of the $0.54 per gallon tariff imposed by the U.S. government on ethanol imports. This is an example of a(n) _____ tariff.
a. specific
b. preferential
c. ad valorem
d. custom
e. generalized