A factory has a linear cost function f(x)=ax+b, where b represents fixed costs and a represents the labor and material costs of making one item, both in thousands of dollars.
If property taxes (part of the fixed costs) are decreased by $12,000 per year, what effect does this have on the graph of the cost function?

a.
shifts to the left by 12 units
b.
shifts to the right by 12 units
c. shifts upward by 12 units
d. shifts downward by 12 units