For users who only want to solicit one specific pledge amount and nothing different

I've seen some users asking for directions on offering just one reward (i.e. $100 pledge only, and nothing different), in order to pre-check that pesky radio button: in template-functions.php, find the following code and add what I've made bold.

if(wdf_has_rewards($post_id) && isset($meta['wdf_levels'][0])) {
$content .= apply_filters('wdf_before_rewards_title','');
$level = maybe_unserialize($meta['wdf_levels'][0]);
foreach($level as $index => $data) {
$content .= '
<div class="wdf_reward_item">
<div class="wdf_reward_choice"><input type="radio" name="wdf_reward" checked=true value="'.$index.'" /><span class="wdf_level_amount" rel="'.$data['amount'].'">'.$wdf->format_currency('',$data['amount']).'</span></div>
<div class="wdf_reward_description">'.$data['description'].'</div>