{code}

//method one
$('#btn_next, #input_answer, #input_left').attr({'disabled':true});
$('#btn_next, #input_answer, #input_left').attr({'disabled':false});

//method two
$('#btn_next, #input_answer, #input_left').attr({'disabled':'disable'});
$('#btn_next, #input_answer, #input_left').removeAttr('disabled');

{Boy Said}

one from myself, the next one from yangmls, I thought the second method is good than the first one, and the method add attr and remove is good. thanks yangmls again.^&^