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

The refrigerator door contains iron of the following statements best explains why magnets usually stick to a refrigerator door.
Which of the following statements best explains why magnets usually stick to a refrigerator door?

The male reproductive system consists of structures that produce sperm and secrete testosterone.
What does the male reproductive system consist of?

The base of a nucleotide within dna consists of nitrogen.

What does the base of a nucleotide within dna consist of?