B
Below is a question with multiple-choice answers. Choose the correct option based on your reasoning.

### Question:
Which of the following statements best explains why magnets usually stick to a refrigerator door?

### Choices:
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.

### Answer:
