The zeroes of the function $f(x)=x^{2}-a x+a$ are integers. What is the sum of the possible values of $a$? (Adapted From 2015 AMC 10A Problems, Question \#23) 
选项为:A:$$2$$  B:$$4$$  C:$$6$$  D:$$8$$  E:$$7$$  请回答其中一个选项.