Keep in mind, the <em>d</em><em>o</em><em>m</em><em>a</em><em>i</em><em>n</em> is a set of <em>x-</em><em>values</em><em> </em>so determined by x-axis, the horizontal one.
In your case the domain appears to be any x less than or equal to zero, so thirdoption.
