Here's a problem to solve: An air mass in a valley travels up a mountainside. What causes the movement of this air?

Among the 4 following options, which is the correct answer?

- A: tidal pull of the moon's gravity
 
- B: evaporation of water from soil in the valley
 
- C: warming by solar energy re-radiated from the ground
 
- D: cooling effect of ice crystals in the air over the mountain
 