keep buttons aligned independent of text length

Question

how to keep a button position unaffected by paragraph size (which can be 3 or 4 lines)?

html:

<div class='container'>
    <div class='row'>
            <div class='col-sm-4'>
                <div class='card-block'>
                    <img class='card-img-top' src='img/sun.png'>
                    <h2 class='card-title text-center'><a href='POST'>
                    TITLE</a></h2>
                    <p class='card-text preview'>POST PREVIEW</p>
                    <div class='text-center'>
                        <a class='btn btn-primary' href='POST'>Read More</a>
                    </div>
                </div>
            </div>
    </div>
</div>  

css:

.col-sm-4 {
margin-bottom: 50px;
}

.card-img-top {
margin-bottom: 25px;
}

.preview {
text-align: justify;
}

screenshot: