4.23 求大神讲解哪错了。

   static void Main(string[] args)
        {
      int [,] a  = new int  [6,5];

      for (int i = 0; i < 6; i ++ )
      {
          Console.WriteLine("请输入第{0}个同学的数学成绩",i +1);
          int  sx =  Convert .ToInt32(   Console.ReadLine());
          Console.WriteLine("请输入第{0}个同学的语文成绩", i + 1);
          int  yw =   Convert .ToInt32 ( Console.ReadLine());
          Console.WriteLine("请输入第{0}个同学的英语成绩",i +1);
          int  yy =     Convert .ToInt32 (    Console.ReadLine());
         a[i, 0]=i +1;
             a[i, 1]=sx ;
          a[i, 2]=yw ;
          a[i, 3]=yy ;
          a[i, 4] = sx + yw + yy;
      }
      for (int i = 1; i <= 5; i ++ )
      {
          for (int j = 1; j <= 6 - i; j ++ )
          {
              if (a[j ,4]>a [j -1 ,4])

              {
                  int xyw = a[j, 1];
                  a[j, 1] = a[j - 1, 1];
                  xyw=a [j-1, 1] ;

                    int xsx = a[j, 2];
                  a[j, 2] = a[j - 1, 2];
                  xsx=a [j-1 ,2] ;

                    int xyy = a[j, 3];
                  a[j, 3] = a[j - 1, 3];
                  a [j-1, 3]=xyy ;

                  int xzf = a[j, 4];
                  a[j, 4] = a[j - 1, 4];
                  a[j - 1, 4] = xzf;

              }
          }
      }
      Console.WriteLine("学号\t语文\t数学\t英语\t总分\t名次");
      for (int i = 0; i < 6; i ++ )
      {
          Console.WriteLine("{0}\t{1}\t{2}\t{3}\t{4}\t{5}",a [i ,0],a [i ,1],a [i ,2],a [i ,3],a [i ,4],i +1);
      }

                   }
                }
            }