In between two point charges a conducting spherical shell having charge $\text{Q}$ is placed as shown in the diagram. Assuming all charges to be positive, mark out the correct statement.

                 

A solid spherical conducting shell has inner radius $a$ and outer radius $2a$. At the center of the shell is located a point charge $+Q$. What must the excess charge of the shell be in order for the charge density on the inner and outer surfaces of the shell to be exactly equal?