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某程序功能如下：单击“输出”按钮 Command1，在列表框 List1 中按序输出{a,b,c,d}的所有子集。程序运行结果如图所示。实现上述功能的 VB 程序如下，请在划线处填入合适的代码。

Private Sub Command1_Click()
Dim s As String, p As String, d(1 To 100) As String, temp As String
Dim i As Integer, j As Integer, t As Integer
s = "abcd"
k = 0
For i = 0 To 15
t = i: p = ""
For j = 1 To 4
If t Mod 2 = 1 Then p = p & Mid(s, j, 1)
（1）      
Next j
k = k + 1
d(k) = p
Next i
For i = 1 To k - 1
t = i
For j = i + 1 To k
If （2）  Then
t = j
End If
Next j
If t <> i Then
temp = d(t): d(t) = d(i):     （3）     
End If
Next i
For i = 1 To k
p = "{"
For j = 1 To Len(d(i))
（4）    
If j <> Len(d(i)) Then
p = p & ","
Next j
p = p & "}" List1．AddItem p
Next i
End Sub
(1)____________
(2)____________
(3)____________
(4)____________