7. Rewrite the following propositions symbolically with the quantifiers explicit and in the correct place: *a) There is a unique line parallel to a given line through a point not on the given line. b) There is a unique point the same distance from any three given points of the plane W. (Denote the distance between P and Q as d(P. 9).) c) Every natural number can be written as the sum of the squares of four integers. d) Two real numbers are equal iff the absolute value of their difference is less than any positive number