C# と VB.NET の質問掲示板

お世話になります。
以下12行目のコメントを外すと（シート追加）エクセルのプロセスが閉じなくなります。
解決方法をご教授頂けると助かります。
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Imports Microsoft.Office.Interop
Public Class Form1
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        Dim xlApplication As New Excel.Application
        Dim xlbooks As Excel.Workbooks = xlApplication.Workbooks
        Dim xlBook As Excel.Workbook = xlbooks.Open("c:\test.xlsx")
        Dim xlSheets As Excel.Sheets = xlBook.Worksheets
        Dim xlSheet As Excel.Worksheet = xlSheets(1)
        Dim xlCells As Excel.Range = xlSheet.Cells
        'xlSheet = xlSheets.Add()

        System.Runtime.InteropServices.Marshal.ReleaseComObject(xlCells)
        System.Runtime.InteropServices.Marshal.ReleaseComObject(xlSheet)
        System.Runtime.InteropServices.Marshal.ReleaseComObject(xlSheets)
        xlBook.Close(SaveChanges:=False)
        System.Runtime.InteropServices.Marshal.ReleaseComObject(xlBook)
        System.Runtime.InteropServices.Marshal.ReleaseComObject(xlbooks)
        xlApplication.Quit()
        System.Runtime.InteropServices.Marshal.ReleaseComObject(xlApplication)
    End Sub

End Class