<form>
<select name="a" onchange="javascript:alert(this.value);">
<option value="1">A</option>
<option value="2">B</option>
<option value="3">C</option>
</select>
</form>
<select name="a" onchange="javascript:alert(this.value);">
<option value="1">A</option>
<option value="2">B</option>
<option value="3">C</option>
</select>
</form>
<form>
<select name= "a" onchange= "javascript:document.getElementById('show').innerHTML = '选'+this.options[this.selectedIndex].text+'的值是:'+this.value;">
<option value= "1">A</option>
<option value= "2">B</option>
<option value= "3">C</option>
</select>
</form>
<div id="show"></div>