How to generate 4 individual CSV files from an Excel 2003 workbook, in C#, using VS2010?

I am developing a C# web application using VS2010.

I have a directory titled U:\Visual Studio 2010\Projects\SMRWebForm\SMRWebForm\
that contains Excel 2003 workbooks made up of multiple worksheets in each workbook.

I would like to create a CSV file for each of 4 particular worksheets in each of the workbooks. The workbooks have other worksheets but I am only interested in creating
CSV files for 4 particular worksheets in each of the workbooks.

The 4 worksheets in each workbook that I would like to create CSV files for have the following worksheet names:

Bank Credits (A)
Bank Debits (B)
Book Debits (C)
Book Credits (D)

I use the following code in an Import click event to create a CSV file:

 protected void Import_Click(object sender, EventArgs e)
        {
            ExcelHelper.ConvertExcelToCsv(@"U:\Visual Studio 2010\Projects\SMRWebForm\SMRWebForm\105 Internal  08-28-2013.xls", @"U:\Visual Studio 2010\Projects\SMRWebForm\SMRWebForm\105 Internal  08-28-2013.csv", 7);

            string fileName = @"U:\Visual Studio 2010\Projects\SMRWebForm\SMRWebForm\105 Internal  08-28-2013.csv";


 public static class ExcelHelper
        {
            public static void ConvertExcelToCsv(string source, string destination, int sheetNumber = 1)
            {
                if (File.Exists(destination)) File.Delete(destination);
                Application xl = new Application();
                try
                {
                    Workbook workbook = xl.Workbooks.Open(source);
                    Worksheet ws = (Worksheet)workbook.Sheets[sheetNumber];
                    ws.SaveAs(destination, XlFileFormat.xlCSV);
                    Marshal.ReleaseComObject(ws);
                }
                finally
                {
                    xl.DisplayAlerts = false;
                    xl.Quit();
                    Marshal.ReleaseComObject(xl);
                }
            }
        }

Do you know how this C# code could be revised to read ALL of the XLS files in the directory U:\Visual Studio 2010\Projects\SMRWebForm\SMRWebForm\
and for each XLS file read, create 4 individual CSV files?

For this XLS file titled 105 Internal  08-28-2013.xls
I would like to generate 4 individual CSV files titled:

105_Bank Credits (A).csv
105_Bank Debits (B).csv
105_Book Debits (C).csv
105_Book Credits (D).csv

And then assume the next XLS file in the directory is titled "222 Internal  08-28-2013.xls"

I would like to generate 4 CSV files titled as follows:

222_Bank Credits (A).csv
222_Bank Debits (B).csv
222_Book Debits (C).csv
222_Book Credits (D).csv

Thus, if there are 20 XLS files in the directory, the application should generated 80 CSV files.

I have attached a sample workbook.
105-Internal--08-28-2013.xls