分析可知：$$\square =\bigcirc +\bigcirc =2\bigcirc $$，  把$$\square =2\bigcirc $$代入$$\bigcirc +\square +\square =30$$中得：  $$\bigcirc +2\bigcirc +2\bigcirc =5\bigcirc $$，$$5\bigcirc =30$$，  所以$$\bigcirc =30\div 5=6$$，  那么$$\square =2\bigcirc =2\times 6=12$$，  由此可知，$$\square $$与$$\bigcirc $$的差为$$12-6=6$$．  故选$$\text{C}$$． 
$$\square $$和$$\bigcirc $$各代表一个数，$$\square =\bigcirc +\bigcirc $$，$$\bigcirc +\square +\square =30$$，求$$\square $$与$$\bigcirc $$的差是． 
a: $$3$$ 
b: $$4$$ 
c: $$6$$ 
d: $$8$$ 