Perpendicular BisectorImmersive Reader A-Lines that intersect at 90* angles
B- A line, ray or segment that divides a segment into two congruent segments
C- A segment, ray, line or a plane that is perpendicular to a segment at its midpoint
D- Two angles whose measure have a sum of 90*

then

there is only one way to arrange all the eight relative to each other because they are not distinct. we have

now there are nine places to place an (and not more than one in one place).   because we have four symbols, we need to choose four of these nine places to insert an . order does not matter because the are not distinct.

thus there are 1 · c(9,4) ways, or, if we calculate it,

thus there are 126 ways to form such "words."