Script does not work when I put it in js file

I need to make a form field appear according to a choice made by the user. I made (or rather copied and modified) a script to that purpose :

<script> <!-- conditional for cross-test not done -->
                ("#CT").change(function() {
                        if (("#IfCrossTestFalseBox").hide();
                        }else {
                            $("#IfCrossTestFalseBox").show();
                        } 
                    });
                });
            </script>

Basically, when an option is selected, an additional form field appears or not.

Also, this JS query is necessary for this script to work :

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.5.1/jquery.min.js"></script>

I actually have two problems with this method :

• First and foremost : this script only seems to work within the html body with tag. When I put it in a separate js file, it does not work. I suspect it is because I did not name a function for it, but I do not know how to properly name it and trigger it after that ?
• Secondly, when another choice is made midway, and previous (conditional) form fields are filled, they are not deleted. How do I reset these particular values ? Or is there another method that I could use instead maybe ?

Cheers ! =)