Search dates range

[Osmarls]

Hi friends, I have a file where I have several spreadsheets with names of products in each worksheet has the clouna A: Date B: Invoice, C: concentration, how can I do a search of a date range in column A(01/01/2011 at 30/01/2011), leading consider the column C, if any value is entered.
I need to know how many values ​​you entered in column C will seem

Thanks

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Osmarls