Alert message on select option

I have a form that I want to pop up an alert message is someone selects the letter “A” from my select menu. I tried all different stuff and cant get the alert message to pop up when “A” is selected. I tried :
if(document.myform.myvalue.value = “A”)

and it popped up alert message on all selections.

Here is the whole form part where I tried:

function check()
{

if(document.myform.myvalue.value == “A”)
{
alert(“Dont select the letter A”)
return false

}

<cfform name=”myform” action=”dosomething.cfm” onSubmit=”return check();”>

<select name=”myvalue” size=”1″>
<option value=”A” selected>A</option>
<option value=”b”>b</option>
<option value=”c”>c</option>

</select>

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