Given a scientific passage and an answer, generate a question for the given answer.

[Q]: Passage: Zinc reacting with hydrochloric acid produces bubbles of hydrogen gas. Answer: hydrogen
[A]: Zinc reacting with hydrochloric acid produces bubbles of which gas?

[Q]: Passage: Frozen precipitation may fall as snow, sleet, or freezing rain. Answer: snow, sleet, freezing rain
[A]: What three forms does frozen precipitation take?

[Q]: Passage: Bones become increasingly ossified and grow larger during fetal development, childhood, and adolescence. When skeletal maturity is reached at about age 20, no additional growth in bone length can occur. Answer: 20
[A]:
At what age is skeletal maturity reached by humans?