Answer:
A vertical line has an infinite or undefined slope since the denominator is zero.
Explanation:
Parallel lines by definition refers to lines that never intersect or meet since they have identical slopes. The slope of line is defined as;
(change in y)/(change in x)
For a vertical line, the y values are changing while the x values remain constant. The slope of this line will thus have a zero value in the denominator implying that its slope will not defined or will be infinity.