Two metal spheres of same material and radius r are in contact with each other. The gravitational force of attraction between the spheres is proportional to

• A.  \[{{r}^{4}}\]    
• B.  \[1/{{r}^{3}}\]  
• C.  \[{{r}^{3}}\]                      
• D.  \[{{r}^{2}}\]