Given the polynomial f(x) = x4 + 3x3 − 3x2 − x, which of the following is true? (1 point)

(x + 1) is a factor since f(−1) = 0.
(x − 1) is a factor since f(−1) = 0.
(x + 1) is a factor since f(1) = 0.
(x − 1) is a factor since f(1) = 0.