js 实现多项选择题目

<script language=javascript>
function changeselect(selectname)
{
answer.innerText=""
for(i=0;i<=yesanswer.length-1;i++)
{
if(yesanswer[i].checked)answer.innerText=answer.innerText+yesanswer[i].value
}
}
</script>
鞋垫具有哪些功能　（<span id=answer></span>）<br>
<input name="yesanswer" type="checkbox" value="A" onClick="changeselect(this)">
                                A.消除脚气脚臭、风湿光节炎痛<br>
<input name="yesanswer" type="checkbox" value="B" onClick="changeselect(this)">
                                B.促进循环         <br>
<input name="yesanswer" type="checkbox" value="C" onClick="changeselect(this)">
                                C.A，B答案均是                                              <br>
<input name="yesanswer" type="checkbox" value="D" onClick="changeselect(this)">
                                D.能强筋健力