Dropdown Menu Search

["Andy" <andy@xxxxxxxx>]

My search page has four dropdown menus.
If a visitor makes a selection from all four dropdowns, my results page 
recordset would be easy, select * from table where fieldname1 = dropdown 
value 1 AND fieldname2 = dropdown value2 AND etc....
But what if the visitor only makes a selection from two or three of the 
dropdown menus?
How would i write the recordet?
Im using classing ASP, Access 2008 and DW 8

Thanks
Andy