ITMC 2018 Grade 9 National Preliminary set01 Question 2

Question 2

A linear polynomial has degree 1 and form $ax+b$, where $x$ is a variable and $a,b$ are real constants. Examples: $2x+1,x+20,-x-10,3x-10,\frac12x-1$. Which polynomial cannot be written as a product of two linear polynomials?

$x^2-x-1$
$x^2-1$
$x^2-x+1$
$x^2-2x-2$
$x^2+x-1$