求解。。红色那段代码中的elseif语句不执行得到的y值只有1和3为什么

<input name="option" type="radio" value="1" checked="checked"/>
<input type="radio" name="option" value="2" />
<input type="radio" name="option" value="3" />
<input name="option1" type="radio" value="1" />        
<input type="radio" name="option2" value="2" />
<input type="radio" name="option3" value="3" />
<script language="vbscript">
function pk()
document.getElementById("option1").checked = false
document.getElementById("option2").checked = false
document.getElementById("option3").checked = false
dim x,y,a,w
randomize
x = int(rnd()*3)+1
select case x
case 1
document.getElementById("option1").checked = true
case 2
document.getElementById("option2").checked = true
case 3
document.getElementById("option3").checked = true
end select
if document.getElementById("option").value =1 and document.getElementById("option").checked = true then
    y=1
elseif document.getElementById("option").value =1 and document.getElementById("option").checked = true then
    y=2
else
    y=3
end if
if  x - y = -1 or  x - y = 2  then
msgbox "恭喜你，你赢了 "
elseif x - y = 0 then
msgbox "打平手"
else
msgbox "电脑胜出"
end if
end function
</script>