bootstrap设计响应式按钮（浏览器变大和变小时按钮变化）

<div class="col-sm-4 text-right">
<button type="submit" class="btn btn-primary hidden-xs">Log In</button>
<button type="submit" class="btn btn-primary btn-block btn-lg visible-xs bk-margin-top-20">Log In</button>
</div>

/*
log in 的btn
*/
.bk-margin-top-20 {
margin-top: 20px !important;
}
.btn-block {
display: block;
width: 100%;
}
.btn-lg {
border-radius: 3px;
padding: 12px 22px;
font-style: 17px;
line-height: 1.33;
}