How can I preselect a radio button using javascript or jquery?

I know this is probably easy, but I just can't figure it out.

I am working with a SAAS shopping cart solution so I can't adjust the form code or use PHP, so I need to use javascript or jquery.

In this cart, I have a form....

<form method="post" action="cart.php" id="productDetailsAddToCartForm">  
Please Select a Bottle Size:  
<ul>  
<li><label><input name="variation[1]" type="radio" class="RadioButton" value="30"  /> 11oz</label></li> 
<li><label><input name="variation[1]" type="radio" class="RadioButton" value="31"  /> 33oz </label>  
</li>  
</ul>  
</form>

And I need to preselect the first radio button using jquery or javascript.

Can you help?

You can use the attr method with checked like this:

$('.RadioButton:first').attr('checked', true);

Or

$('.RadioButton:first').attr('checked', 'checked');

Note that :first will select the first radio button there.

$(':input:radio:eq(0)').attr('checked', 'checked');

This will check the first radio box on the page.