下列程序的执行结果为
Private Sub Command1_Click( )
Dim FirStr As String
FirSt = "abcdef"
Print Pat(FirStr)
End Sub
Private Function Pat( xStr As String) As String
Dim tempStr As String, strLen As Integer
tempStr =""
strLen = Len(xStr)
i=1
Do While i＜=Len(xStr)-3
tempStr = tempStr + Mid(xStr, i, 1 ) + Mid(xStr, strLen - i + 1, 1 )
i=i+1
Loop
Pat = tempStr
End Function

A．abcdef

B．afbecd

C．fedcba

D．defabc

简述宪法典的内容结构。