Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Ex Input:
The refrigerator door contains iron of the following statements best explains why magnets usually stick to a refrigerator door.

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


Ex Input:
Deep currents are caused by differences in density at the top and bottom of the ocean.

Ex Output:
What is caused by differences in density at the top and bottom of the ocean?


Ex Input:
Weeds are plants that grow where people don't want them to and can take up space and use resources which hinders growth of more desirable plants.

Ex Output:

What are plants that grow where people don't want them to and can take up space and use resources which hinders growth of more desirable plants?