I have a form for signing someone up for email alerts. The user has several options for what type of alerts to get. One of these options is to be emailed when a specific interest rate becomes available for a loan. If they want this type of email they check a check box and then select the rate they want from a list. If they check the box for this type of alert, selecting a rate becomes required.

What I want to do is make it so that if someone selects a rate from the list, it will automatically check the check box. I want it to do it in an AJAX way so that right when they select a rate form the list, they will see the check box become marked.

Here is the form I have set up (This form is much longer with more alert options, but I cut it down for space):

HTML Code:
<form id="alerts" action="/about/mortgage-rate-alerts" method="post">
    <fieldset>
        <legend>Sign up for alerts below:</legend>

        <div>  
            <!-- BEGIN email_address -->  
            <p><label for="email_address">E-mail address:</label> <input class="text" id="email_address" tabindex="1000" type="text" name="alerts[emailAddress]"></p>
                
                
            <!-- END email_preferences -->
        </div>    
            
            <!--BEGIN ALERTS_RATE -->
            <div class="rate_container">  
                    <h2><input value="" type="hidden" name="alerts[rateAlert]">
<label><input tabindex="1000" class="alert_check_box" name="alerts[rateAlert]" type="checkbox" value="1"> Rate Alert</label></h2>

                    <div id="rate_alert_headline">
                        	<p>Tell us exactly what rate youíre looking for and well get in touch when itís available.</p>
                    </div>
                    <p>
                        What rate do you want?
                        <select class="select" tabindex="1000" name="alerts[rate]">
<option value="">Choose Rate</option>

<option value="rate">rate</option>
<option value="rate">rate</option>
<option value="rate">rate</option>

</select>

                    </p>
                    
                    	
            </div>    
            
            <div><input id="submit_form" value="Complete" tabindex="1000" src="sign_me_up.png" type="image" name="alerts[submit]"></div>
    </fieldset>
</form>