Here's a problem to solve: Which of the following statements best explains why magnets usually stick to a refrigerator door?

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

- A: The refrigerator door is smooth.
 
- B: The refrigerator door contains iron.
 
- C: The refrigerator door is a good conductor.
 
- D: The refrigerator door has electric wires in it.
 