What causes valley air to move up surrounding mountains during the day? The Sun warms the air in the valley more than it does the mountains.
The Coriolis Force moves the winds up the sides of hills and mountains.
The Sun warms the tops of hills and mountains more than it does the valley.
The valley air is denser because is of its lower elevation.

so the third one

Explanation:

During the day, the sun heats up mountain air rapidly while the valley remains relatively cooler. Convection causes it to rise, causing a valley breeze. At night, the process is reversed. During the night the slopes get cooled and the dense air descends into the valley as the mountain wind.

The Sun warms the tops of hills and mountains more than it does the valley.

Explanation:

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.