Answers for "a bootstrap file for rows and coumns"

columns center bootstrap 3

<div class="row">
    <div class="col-md-2 col-md-offset-5"></div>
</div>

bootstrap screen sizes

The Bootstrap grid system has four classes:
xs (for phones - screens less than 768px wide)
sm (for tablets - screens equal to or greater than 768px wide)
md (for small laptops - screens equal to or greater than 992px wide)
lg (for laptops and desktops - screens equal to or greater than 1200px wide)