Кнопка в div при прокрутке прячется за другим div

<div class = "modal fade" id = "myModal" tabindex = "-1" role = "dialog" aria-labelledby = "myModalLabel" aria-hidden = "true">
    <div class = "modal-dialog">
        <div class = "modal-content">
            <div>
                <button type = "button" class = "close" style = "color:#FFFFFF;" data-dismiss = "modal" aria-hidden = "true">&times; </button>
            </div>
            
            <div class = "container content-box">
                <div style = "padding-top:20px;" class = "col-sm-6">
                    <h3 class = "rv1">Fed up with Staff Nurse Entrance Exam?</h3 ><h1 class = "rv2">Download <span style = "color:#ce3328">Nurses Pulse</span> &amp; Prepare for the exam.</h1>
                    <p class = "rv3">NursesPulse is a mobile application for those who are preparing for the staff nurse entrance exam for abroad.
                                                     With NursesPulse you will get the notification of vacancies,apply for the exams and practice for the exams.</p>
                    <a style = "background-color:#00b02f;margin-bottom:20px;" class = "btn btn-success btn-lg rv4" href = "https://play.google.com/store/apps/details?id=com.bluroe.nursespulse"><i class = "fa fa-android" aria-hidden = "true"></i> Download Android Application</a>
                </div>

                <div class = "col-sm-6 text-center">
                    <img class = "image" src = "images/phone.png" />
                </div>
            </div>

            <div class = "modal-footer">
                <button type = "button" class = "btn btn-default" data-dismiss = "modal">Close</button>
            </div>
        </div>
    </div>
</div>

.popup
{
    position: absolute;
    top: 0px;
    left:40%;
    z-index: 1030;
}
.isAnswerStyle{
    color: #090;
    font-weight: bold;
}
#img-upoloader{
    padding-bottom: 10px;
}
.bg{
   background: url("images/bg.jpg") no-repeat center top fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}
.content-box {
    display: table;
    padding-top: 100px;
    color: #FFF;
    font-family: 'Raleway', sans-serif;
}
.content-box p{
    color: #EEE;
}