A convex lens makes a real image 4 cm long on a screen. When the lens is shifted to a new position without disturbing the object, we again get a real image on the screen which is 16 cm tall. The length of the object must be 

• A

  $\frac{1}{4}\mathrm{cm}$

• B

  8 cm

• C

  12 cm

• D

   20 cm