Suppose that the production of $500,000 worth of steel in the United States requires $100,000 worth of iron ore. The U.S. nominal tariff rates for importing these goods are 15 percent for steel and 5 percent for iron ore. Given this information, the effective rate of protection for the U.S. steel industry is approximately: ( )
选项：

A:6 percent
B:12 percent
C:18 percent
D:24 percent