这种验证码怎么识别?后面的What is 1 + 3 不是固定的
<div class="form-group">
<div class="col-lg-12">
<div class="input-group input-icon">
<span class="input-group-addon input-group-lg"><i class="fa fa-check"></i></span>
<input type="text" id="registerAnswer" class="form-control input-lg" placeholder="What is 1 + 3?">
</div>
</div>
</div>