The area of a circle is increased by 300%. By what percent has the radius of the circle increased?
Choose the correct option:
A) 400 %  B) 200 %  C) 100 %  D) 600 %  E) 800 %

Rationale: "the area of the circle is increased by 300 % , thus the area is increased 4 times . the area of a circle it proportional to the square of the radius ( area = π r ^ 2 ) , therefore the radius must increase 2 times ( diameter increase 2 times = area increase 4 times ) , which is increase by 100 % . answer : c"
Answer: C