The refrigerator door is smooth. The refrigerator door contains iron. The refrigerator door is a good conductor. The refrigerator door has electric wires in it.
I gave my students this multiple choice question: Which of the following statements best explains why magnets usually stick to a refrigerator door?

Only one answer is correct among these 4 choices:
- The refrigerator door is smooth.
- The refrigerator door contains iron.
- The refrigerator door is a good conductor.
- The refrigerator door has electric wires in it.

Could you tell me which one is correct?

The refrigerator door contains iron.