Here's a problem to solve: The average temperatures over a four-month period were 31, 24, 18, and 12 degrees Celsius. What is the median temperature for these four months?

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

- A: 15°C
 
- B: 21°C
 
- C: 27°C
 
- D: 33°C
 