On the xy-plane, the graph of function f has x-intercepts at -3, -1, and 1. which of the following could define f?
a) f(x)= (x-3)(x-1)(x+1)
b) f(x)=(x-3)(x-1)^2
c) f(x)=(x-1)(x+1)(x+3)
d) f(x)=(x+1)^2(x+3)