Q and R are two-digit positive integers that have the same digits but in reverse order. If the positive difference between Q and R is less than 20, what is the greatest possible value of Q minus R? Select the correct option:  A) 15  B) 16  C) 17  D) 18  E) 19

Rationale: "a two - digit integer ` ` ab ' ' can be expressed algebraically as 10 a + b . q - r = ( 10 a + b ) - ( 10 b + a ) = 9 ( a - b ) < 20 . the greatest multiple of 9 which is less than 20 is 18 . the answer is d ."
Answer: D