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求解“幻圆”问题。三个圆相互重叠，形成七个区域，编号为①、②、③、④、⑤、⑥、⑦，如图a所示。将数字1～7填入每个区域中，实现所有数字仅用一次且每个圆内的数字之和相等，如图b所示，圆内数字的和为1 + 4 + 6 + 7 = 2 + 4 + 5 + 7 = 2 + 3 + 6 + 7，求解区域①～⑦中可以填入符合条件数字的所有方案。
图a                  图b
求解思路：在七个区域①、②、③、④、⑤、⑥、⑦上枚举七个数字1、2、3、4、5、6、7的所有排列来进行验证。程序运行界面如图c所示。
图c
请回答下列问题：
（1）根据题意及程序代码，在区域①～⑦中按顺序分别填入7，4，2，1，3，6，5，则该排列________（选填：能/不能）构成“幻圆”。
（2）请在划线处填入合适的代码。
Dim a(1 To 7) As Integer    'a(1)～a(7)依次存储区域①～⑦的值
Private Sub Command1_Click()
Dim i As Integer, n As Integer
Dim y1 As Integer, y2 As Integer, y3 As Integer
For i = 1 To 7
a(i) = i
Next i
n = 0
Do While True
For i = 7 To 2 Step -1
If a(i) = 8 Then
a(i) = 1
___________
End If
Next i
If a(1) = 8 Then Exit Do
If check() Then
y1 = a(1) + a(2) + a(3) + a(4)
y2 = a(2) + a(4) + a(5) + a(6)
y3 = a(3) + a(4) + a(6) + a(7)
If y1 = y2 And ___________ Then
n = n + 1
'在列表List1中，按格式输出a数组元素的值，代码略
End If
End If
a(7) = a(7) + 1
Loop
Label1.Caption = "解法共计" & Str(n) & "种。"
End Sub
Function check() As Boolean
Dim i As Integer, n As Integer, f(1 To 8) As Integer
For i = 1 To 8
f(i) = 0
Next i
For i = 1 To 7
__________
Next i
n = 0
For i = 1 To 7
n = n + f(i)
Next i
If n = 7 Then check = True Else check = False
End Function