There are $$15$$ balls in a box: white balls, red balls and black balls. The number of white balls is $$7$$ times greater than the number of red balls. How many black balls are there in the box? ($$1998$$ Math Kangaroo Problem, Level $$3-4$$, Question \#$$16$$) 
选项为:A:$$1$$  B:$$3$$  C:$$5$$  D:$$7$$  E:$$9$$  请回答其中一个选项.