Check Querystring

Here is what I am doing ... Have a page default.asp which looks at the
url and checks for querystring's. if there are none passed goes to a
function. if there are particular querystrings passed goes to the
respective function.

Now there can be only 2 valid querystrings that can be passed and only
one at a time. How to do i check for querystrings that dont exist and
send them to error page?

for example I have QS(querystring) ID and State. only one can exist at
a time or none. if I pass QS country along with one of those two, it
still goes to that function. Can I look for querystring that shouldnt
be valid in my case and redirect them to error page? Code: