原式$$=1\div 3\times 5\div 5\times 7\div 7\times 9\div 9\times 13\div 13\times 15$$$$=1\div 3\times 15$$$$=5$$． 
计算：$$1\div (3\div 5)\div (5\div 7)\div (7\div 9)\div (9\div 13)\div (13\div 15)=$$． 

a) $$\frac{1}{5}$$ 
b) $$\frac{1}{3}$$ 
c) $$3$$ 
d) $$5$$ 