Two non-intersecting circles, one lying inside another, are of diameters a and b, respectively. The minimum distance between their circumferences is c. The distance between their centres is:

• A.  \[a-b-c\]         
• B.           \[a+b-c\]
• C.  \[\frac{1}{2}\,\left( a-b-c \right)\]      
• D.           \[\frac{1}{2}\,(a-b)-c\]