In 10 years,A will be twice as old as B was 10 years ago.If A is now 9 years older than B,the present age of B is :  Choose one.
A) 19  B) 29  C) 39  D) 49  E) none of these

Rationale: "explanation : let b ' s present age = x years . then , a ' s present age = ( x + 9 ) years . ( x + 9 ) + 10 = 2 ( x - 10 ) = > x + 19 = 2 x - 20 = > x = 39 . answer : c"
Answer: C