If the diameter of circle R is 40% of the diameter of circle S, the area of circle R is what percent of the area of circle S? Choose the correct choice from the following choices
A) 16 %  B) 18 %  C) 20 %  D) 22 %  E) 24 %

Rationale: "let diameter of circle r , dr = 40 and diameter of circle s , ds = 100 radius of circle r , rr = 20 radius of circle s , rs = 50 area of circle r / area of circle s = ( pi * rr ^ 2 ) / ( pi * rs ^ 2 ) = ( 20 / 50 ) ^ 2 = ( 4 / 10 ) ^ 2 = 16 % answer : a"
Answer: A