Answer:
A physical boundary is a natural barrier between two areas. Many times, political boundaries between countries or states form along physical boundaries.
Answer to your question is A
Culture is what feels/is comfortable for a state, country, or city. There isn't an exact culture for all the people on this planet.
I would say at night, when it's cooler. At night, desert temperature lowers due to the lack of water. Deserts cannot support much vegetation, so at night the sand loses all of its heat, making it colder. Sand is a major reason why deserts are hot.