程序输出结果为（）
public class Test {
public static void main(String[] args) {
StringBuffer a = new StringBuffer("A");
StringBuffer b = new StringBuffer("B");
operator(a,b);
System.out.println(a+","+b);
}
public static void operator(StringBuffer x,StringBuffer y){
x.append(y);
y=x;
} 
}

A.A，A
B.A，B
C.AB，AB
D.AB，B