Radio input checked

Radio input checked

I have 2 radio inputs. A,B
I an tring to make A as a selected radio by default. And wants when A get
hide then B should get checked by default.
This is what I am trying. Please take a look.
<div>
<input type="radio" checked="checked" value="0" name="a" />
<strong>A</strong></div>
<div>
<input type="radio" checked="" value="1" name="b" />
<strong>B</strong>
</div>
<input type="text"
onfocus="Javascript:document.forms[1].opt_payment[1].checked=true;"
maxlength="20" value=" " name="" />
Thanks