find the least common denominator for the following rational expressions.
18/r-4 5r/r^2-5r+4 r+7/r^2-4r-5
(r-1)(r-4)(r-5)
(r- 1)(r + 4)(r-5)
(r-1)(r + 1)(r – 4)(r-5)
(r- 1)(r+ 1)(r + 4)(r-5)