Let $a, b$, and $c$ be three distinct one-digit numbers. What is the maximum value of the sum of the roots of the equation $(x-a)(x-b)+(x-b)(x-c)=0$? 
选项为:A:$$15$$  B:$$15.5$$  C:$$16$$  D:$$16.5$$  E:$$17$$  请回答其中一个选项.