有如下代码,下面的哪个项可以插入在指定位置上“//处”()。 public class Abcd{ public long sum(long a,long b){return a+b;} // } A. public int sum(int a,int b){return a+b;} B. public int sum(long a,long b){return 0;} C. abstract int sum(); D. private long sum(long a,long b){return a+b;}