一个关于java的题目,望高手指导一下

Given:
When //do Stuff  is reached,how many objects are eligible for GC.

class  CardBoard {
     Short story = 5;
     CardBoard go(CardBoard cb){
            cb = null;
            return cb;
          }

public static void main(String[] args){
    CardBoard c1 = new CardBoard();
    CardBoard c2 = new CardBoard();
    CardBoard c3 = c1.go(c2);
    c1=null;
    //do Stuff
   }
}

高手帮解释一下,谢谢了.