ACT Math Practice Question: Zeros of a polynomial

Try this ACT math practice question on zeros of a polynomial function:

The zeros of a function are those values of $x$ for which $f(x)=0$. Which of the following is the correct description of the zeros of the cubic polynomial $f(x)=2x^3+8x$ ?

1. $\quad \textrm{No real zeros.}$
2. $\quad \textrm{One real zero.}$
3. $\quad \textrm{Two real zeros.}$
4. $\quad \textrm{Two double zeros.}$
5. $\quad \textrm{Three distinct real zeros.}$