[jQuery] [validate]If the A field is changed, how to call the validate method of B field?

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There is a question: If the A field is changed, how to call the
validate method of B field?

    In my form, there are A and B fields, I have to call B's remote
method, when one of A and B have changed,

    coding:
        $(myForm).validate({
                rules: {
                        B: {
                                remote: function(){
                                        return 
"/validateABExist.do?A="+$("#A").val()
                        }
                messages: {
                        B: {
                                remote: "the combination of A and B is existed"
                        }
                }
        })

        $("#A").change(function() {
                //TODO: How to call B's remote method?

        });