Why is it warmer on the side of a hill than on the valley floor？A．Because warmer air lifts

Why is it warmer on the side of a hill than on the valley floor？

A．Because warmer air lifts up and off the valley floor, especially on clear nights.

B．Because there is difference between the temperature.

C．Because cold wind blows down to the valley.

D．No information is mentioned in the passage.