I am traveling to Nepal and back to USA. Do I need a visa and if yes, what kind of visa?

I am traveling to Nepal and back to USA. I have a layover of 13 hrs while traveling to Nepal in Gongzhou airport and 8 hrs while traveling back to USA but the layover is in two different cities in China while returning to USA. Do I need a visa to travel back to USA and if yes, pleas let me know what kind of visa should I apply for?