Here's a problem to solve: A daphnia population shows a vertical migration from the bottom of a one-meter-deep pond during the day to the surface of the pond at night. To which factor is the daphnia population most likely responding?

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

- A: the pH of the water
 
- B: the clarity of the water
 
- C: the pressure of the water
 
- D: the temperature of the water
 