fabrykant

<form id="search" method="get" action="/search">
<div class="block">
<select id="number" class="span3" name="number">
 <option value="0">Выбрать</option>
 <option value="1">Первый</option>
 <option value="2">Второй</option>
</select>
</div>
<div class="block">
<select id="textID" class="span3" name="textID">
 <option value="0">Выбрать</option>
 <option value="11">Да</option>
 <option value="12">Нет</option>
</select>
</div>
<button id="btn" value="Submit">ответить</button>
</form>

if($("#textID").val() == 0){
  $("#btn").attr("disabled", "disabled");
}
$("#textID").on("change", function(){
  if($("#textID").val() == 0){
  $("#btn").attr("disabled", "disabled");
}else{
  $("#btn").removeAttr("disabled");
}
$("#btn").on("click", function(){
   alert('ответ');
 })
})