combobox rowsource "All" if more than one in list

access 2003

I have a combobox with the below row source. It contains a Union to add
"All units".

How can I edit this to only show "All Units" if there are more than one
selection in the combobox?

i.e.
if combobox rowsource has 3 units then "All Units" would be included.
if combobox rowsource has only 1 unitthen only list the one unit.

SELECT t000GFacts.UnitID, t000GFacts.Unit, t000GFacts.UnitType,
t000GFacts.AEIC, t000GFacts.PlantName, t000GFacts.Unit AS UnitOrder,
t000GFacts.SiteName, t000GFacts.OperatingCompany AS Customer,
t000GFacts.ProjectNo, t000GFacts.SiteID, t000GFacts.PlantID,
t040Project.ProjectID FROM t040Project INNER JOIN (t000GFacts INNER JOIN
t041ProjectDetails ON t000GFacts.UnitID = t041ProjectDetails.UnitID) ON
t040Project.ProjectID = t041ProjectDetails.ProjectID WHERE
(((t040Project.ProjectID) Like [Forms]![f001ProjectReview]![ProjectID]))

UNION SELECT 0 AS UnitID, "All Units" AS Unit, "" AS UnitType, "" AS AEIC,
"" AS PlantName, "" AS UnitOrder, "" AS SiteName, "" AS Customer, "" AS
ProjectNo, "" AS SiteID, "" AS PlantID, t040Project.ProjectID FROM
t040Project INNER JOIN (t000GFacts INNER JOIN t041ProjectDetails ON
t000GFacts.UnitID = t041ProjectDetails.UnitID) ON t040Project.ProjectID =
t041ProjectDetails.ProjectID WHERE (((t040Project.ProjectID) Like
[Forms]![f001ProjectReview]![ProjectID]))
ORDER BY t000GFacts.PlantName, t000GFacts.Unit;



Thanks bunches,

--
deb