how to operate on select box using ajax in struts2?

I am doing a project on struts2 in which i have a jsp page which has two select boxes like

<s:form action="">
    <s:select id="d" name="dist" onchange="block();" list="dist_list" label="District" headerKey="-1" headerValue="--Please Select--"></s:select>
    <s:select id="b" name="block" list="block_list" label="Block" headerKey="-1" headerValue="--Please Select--"></s:select>
    <s:submit></s:submit>
    </s:form>

Now i want to use ajax so that when i select a district value it will retrieve me all the blocks under it by assigning to the block list select box.

so please help me..its urgent ... thanx in advance...