If Then Assistance

[P-chu via AccessMonster.com]

I have a simple form with two drop down combos, a list box and date fields.
If I select the beg/end dates and the first combo (which is cboIndustries) I
select an Industry from the cbo such as education, my list box will then show
me all educational institutions that are working with my company during the
criteria dates selected.

This was working just fine. Now I want to add a second combo box to separate
this down a little tighter for all industries per a specific state ( called
cboState). I want the open of viewing all specific industries or specific
industries associated with specific states. Need assistance as to doing an
If/Then statement and when/where to put it.

Additionally, if the Industry cbo is not selected, but the state cbo is, I do
not want the list box to populate any information till an Industry specific
is selected. Will this work.

cboStates_AfterUpdate
If [cboIndustries] is null
Then [lbxresults] is null
else
If [cboIndustries] is not null
Then [lbxresults] is not null

Of should I use visible and not visible option? I'm missing something or I'm
putting this in the wrong place.

Thanks in advance.
P-Chu