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如果两个质数的差为2，则这两个质数称为孪生质数对；编写VB程序，实现如下功能：单击“显示孪生质数对”按钮Command1，在列表框list1中显示100以内的所有孪生质数对(运行效果如图所示)。

实现上述功能的VB代码如下, 但加框处代码有错，请改正。
Dim a(1 To 50) As Integer
Private Sub Command1_Click()
Dim i As Integer
k =" 0:" i = 3
Do While i <= 100
If   prime(i)   Then      ‘⑴
k =" k" + 1
a(k) = i
End If
i =" i" + 2
Loop
For i =" 2" To k
If   a(i) =" a(i" +1)+2   Then   ‘⑵
List1.AddItem Str(a(i - 1)) + "和" + Str(a(i))
End If
Next i
End Sub
Function prime(x As Integer) As Boolean
prime = False
For i =" 2" To Int(Sqr(x))
If  x Mod i =" 0"  Then
prime = True
Exit For
End If
Next i
End Function