C# 中的二维数组是什么？

int [,] a = new int [2,2] {
   {0, 1} ,
   {4, 5}
};

using System;

namespace ArrayApplication {
   class MyArray {
      static void Main(string[] args) {
         /* an array with 3 rows and 2 columns*/
         int[,] a = new int[3, 2] {{0,0}, {1,2}, {2,4} };
         int i, j;

         /* output each array element's value */
         for (i = 0; i < 3; i++) {

            for (j = 0; j < 2; j++) {
               Console.WriteLine("a[{0},{1}] = {2}", i, j, a[i,j]);
            }
         }
         Console.ReadKey();
      }
   }
}