$('.Btn').click(function(){
var Txt1 = $('.txt1').val();
var Txt2 = $('.txt2').val();
if (Txt1 == '' && Txt2 == '') {
alert('必须填一个')
}else if(Txt1 == '' && Txt2 !== ''){
alert('C')
}else if(Txt1 !== '' && Txt2 == ''){
alert('B')
}else{
alert('A')
}
})