Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
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Question: The refrigerator door contains iron of the following statements best explains why magnets usually stick to a refrigerator door.

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


Question: Filter feeders called rotifers collect digestive and excretory wastes in cloacal bladder primitive organ.

Answer: Filter feeders called rotifers collect digestive and excretory wastes in what primitive organ?


Question: Each silicon atom shares four many valence electrons with other silicon atoms in a crystal.

Answer:
Each silicon atom shares how many valence electrons with other silicon atoms in a crystal?