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 am hesitating between 4 options to answer the following question, which option should I choose?
Question: Which of the following statements best explains why magnets usually stick to a refrigerator door?
Possibilities:
- 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.

The refrigerator door contains iron.