Show textbox on radio button check Options

[DeadSayWhat]

Hi, for an assignment I have a username and password input textbox appear when a radio button is checked and can't figure out how.
Any help would be greatly appreciated.

Currently I have

html
<div class="usernamebox"><label for="username">Username: </label><input type="text" id="username" name="username" placeholder="username"></div>
<div class="passwordbox"><label for="password">Password: </label><input type="password" id="password" name="password" placeholder="password"></div>

java

<script>function display() {if(document.getElementById("signup").checked) {
document.getElementById("username").style.display = "inline";
document.getElementById("password").style.display = "inline";}}</script>

css
.usernamebox{display: none;}
.passwordbox{display: none;}

[CharlesEF]

Where's the HTML for the radio button?