The operation is defined for all integers a and b by the equation ab = (a - 1)(b - 1). If x9 = 160, what is the value of x ?  Choose the correct option:
A) 18
B) 15
C) 17
D) 19
E) 21

Rationale: "ab = ( a - 1 ) ( b - 1 ) x 9 = ( x - 1 ) ( 9 - 1 ) = 160 - - > x - 1 = 20 - - > x = 21 answer : e"
Answer: E