The function $f(x)=3x^2$ is :

increasing in(-∞, -1) ∪ (1, ∞) and decreasing in (-1, 1) decreasing in (-∞, -1) ∪ (1, ∞) and increasing in (-1, 1) increasing in (0, ∞) and decreasing in (-∞, 0) decreasing in (), ∞) and increasing in(-∞, 0)

increasing in (0, ∞) and decreasing in (-∞, 0)