If f(x) = x^2, which of the following describes the graph of f(x - 1)?
a.) the graph of f(x - 1) is a horizontal shift of f(x) = x^2 one unit to the left.
b.) the graph of f(x - 1) is a vertical shift of f(x) = x^2 one unit up.
c.) the graph of f(x - 1) is a horizontal shift of f(x) = x^2 one unit to the right.
d.) the graph of f(x - 1) is a vertical shift of f(x) = x^2 one unit down.