If the directrix is horizontal, then the parabola opens (text{horizontally}/text{vertically})

If the directrix is horizontal, then the parabola opens $\left(\text{horizontally}/\text{vertically}\right)$
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Step 1
Given:
We must find the missing word in the given sentence.
So, Definition of parabola said:
A parabola is another type of conic section generated by the cross section of a cone intersected by a plane.
An equation of the form:
$y=a{x}^{2}+bx+c\left(a\ne 0\right)$
Step 2
The above equation is parabola opening upward if $a>0$ and opening downward if $a<0.$
So, the geometrically definition of parabola is the set of all points in a plane that the equidistant from a fixed line called directrix and a fixed point called the focus.
Step 3
In the standard form of a parabola, if x and y are replaced by $x-h\phantom{\rule{1em}{0ex}}\text{and}\phantom{\rule{1em}{0ex}}y-k$, then the graph of the parabola is shifted h units horizontally and k units vertically. The vertex is (h, k), and the focus is $\left(h,k+p\right).$
The directrix is the line defined by the equation $y=k-p$ and the equation of the parabola is:
${\left(x-h\right)}^{2}=4p\left(y-k\right)$
Step 4
The equation of directrix $y=k-p$ is horizontal. So, the parabola opens towards vertically.
Thus, the missing word is Vertically.