In circle M
M below, diameter
A
C
‾
AC
, chords
A
B
‾
AB
and
B
C
‾
BC
, and radius
M
B
‾
MB
are drawn. Which statement is
not true?

A vertical line is described by the x-coordinate only.  There is no slope involved.  (the slope of a vertical line is undefined.)
For example,
The vertical line that passes through the point (5,2) is x=5, because all points x=5 passes through all points of the form (5,k), where k is any real number.