A sum of money becomes 7/6 of itself in 7 years at a certain rate of simple interest. The rate per annum is?  Choose the correct option.
A) 50 / 13
B) 50 / 17
C) 50 / 19
D) 50 / 21
E) 50 / 23

Rationale: "let sum = x . then , amount = 7 x / 6 s . i . = 7 x / 6 - x = x / 6 ; time = 7 years . rate = ( 100 * x ) / ( x * 6 * 7 ) = 50 / 21 % . answer : d"
Answer: D