闭包问题

closure-3.js

function fun(n,o) {
  console.log(o)
  return {
    fun:function(m){
      return fun(m,n);
    }
  };
}
var a = fun(0);  a.fun(1);  a.fun(2);  a.fun(3);//undefined,0 ,0 ,0
var b = fun(0).fun(1).fun(2).fun(3);//undefined,0,1,2

// c.fun == function(m) {return fun(m,n)} , n的值通过闭包获得1
var c = fun(0).fun(1);  c.fun(2);  c.fun(3);//undefined,0,1,1
//问:三行a,b,c的console.log结果分别是什么？