The zeroes of the function $f(x)=x^{2}-a x+2 a$ are integers. What is the sum of the possible values of $a$? (2015 AMC 10A Problems, Question \#23) 
选项为:A:$$7$$  B:$$8$$  C:$$16$$  D:$$18$$  E:$$18$$  请回答其中一个选项.