If the simple interest on a sum of money for 2 years at 5% per annum is $50, what is the compound interest on the same at the same rate and for the same time?  Choose the correct answer
A) $ 50
B) $ 51.5
C) $ 52
D) $ 52.5
E) $ 51.25

Rationale: "sum = 50 x 100 / ( 2 x 5 ) = 500 = 500 x ( 1 + 5 / 100 ) ^ 2 = 500 x 21 / 20 x 21 / 20 = 551.25 . c . i = 551.25 - 500 = 51.25 . answer e ) $ 51.25"
Answer: E