HTML triggering events ( LONG)?

i am creating a form for my website and I need a method to make a drop down box appear according to the choice of radio buttons

here is my code so far for the radio buttons and the drop down boxes:-


<!-- sizes radio buttons -->

Size:
<br>
<input type="radio" name="size" value="Adult sizes"> Adult sizes ------------------------->Radio Button A
<input type="radio" name="size" value="Child/Teen sizes"> Child/Teen sizes ---------->Radio Button B
<br> <br>



<!-- drop down boxes -->


<select> ----------------------------------------------------------------------------------------------------->Drop Down Box A
<option value="Please Select">Please select...</options>
<option value="3-4">3-4</option>
<option value="4-5">4-5</option>
<option value="5-6">5-6</option>
<option value="6-7">6-7</option>
<option value="7-8">7-8</option>
<option value="8-9">8-9</option>
<option value="9-10">9-10</option>
<option value="10-11">10-11</option>
<option value="11-12">11-12</option>
<option value="12-13">12-13</option>
<option value="13-14">13-14</option>
<option value="14-15">14-15</option>
<option value="15-16">15-16</option>
</select>

<select> ---------------------------------------------------------------------------------------------------->Drop Down Box B
<option value="Please select">Please Select...</option>
<option value="Small">Small</option>
<option value="Medium">Medium</option>
<option value="Large">Large</option>
<option value="Extra Large">Extra Large</option>
</select>




I know its long but its needed!
So the story is i need a trigger to make Drop Down Box A appear when Radio Button A is selected and vise versa.

regards
no I need a trigger in HTML only please is there not a code or something
btw i am using weebly website editor for the main chunk of the site should i put the javascript in the footer? it gives me the option to do that
 
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