This is math, not English, but okay xD
"Two intersecting lines are perpendicular if and only if they form four 90 degree angles" would be my answer.
Some persons of a desponding spirit are in great concern about that vast number of poor people, who are aged, diseased, or maimed . . . it is very well known, that they are every day dying . . . as fast as can be reasonably expected.
[apex]
