$A$ small object is attached to a light string which passes through a hollow tube. The tube is held by one hand and the string by the other. The object is set into rotation in a circle of radius $r_1$. The string is then pulled down,shortening the radius of the circle to $r_2$. The ratio of the new kinetic energy to the original kinetic energy is

• A
  $\frac{r_1}{r_2}$
• B
  $1$
• C
  $\left(\frac{r_1}{r_2}\right)^2$
• D
  $\left(\frac{r_2}{r_1}\right)^2$