The function f(x) = -1/3x2 is shifted down one unit. Which of the following function equations represents this translation?

To represent this translation take into account that a shift down 1 unit is to substract one unit to the initial expression. It means, the expression that represents this translation is:
[tex]f(x)=-\frac{1}{3}x^2-1[/tex]The right answer is C.