When viewing with a smaller width screen or mobile device, the bottom two columns should be stacked horizontally. You can achieve this by changing the class on the two divs from:
Nicely done Sura. Could you please check out my tribute page here Abulcasis Tribute Page and give feedback. Thanks.
But xs-extra small is sm-small?
Your project looks good. One thing I would do is add some padding to the left and right margins of the screen. There is not much space for your content to breathe. Lack of whitespace can cause things to feel muddled and hard to read.
I liked it. If you want to do more on it, you can add some padding, text-justify(for the paragraphs),and
center-align the text in copyright section. And for responsive design, small fixes for the big image and others by providing some padding.
col-sm is just small. col-xs is extra small. I recommend using the col-sm-6