vb的一个图形题，求大虾详解，感激不尽

1．编写程序，使用双循环输出下列三角形
                       D D D D D D D
                       C C C C C
                       B B B
                       A
解：
   Private Sub Button1_Click
    ( ByVal sender As System.Object, ByVal e As System.EventArgs)
      Handles Button1.Click
      Dim i, j As Integer
      Dim str1 As String
      Dim p As String = "D"
       For i = 1 To 4
        For j = 1 To 20 + 2 * i
         str1 = str1 + " "
         Next
          For j = 1 To 9 - i * 2
           str1 = str1 + p + " "
          Next
           str1 = str1 + Chr(10)
           p = Chr(Asc(p) - 1)
          Next
        MsgBox(str1)
   End Sub
那个i，j有什么用，完全看不懂这个题。跪求哪个好心人解释解释