【select】
<!DOCTYPE html>
<html>
<body>
<select id="box1">
<option value="1">1</option>
<option value="2">2</option>
</select>
<select id="box2">
<option value="1">1</option>
<option value="2">2</option>
</select>
<script type='text/javascript'>
document.getElementById("box1").onchange = function(){
document.getElementById("box2").value = this.value;
};
</script>
</body>
</html>
