The Element Essential for Gas Exchange
Puzzle 15 of Vienna Hard Pack
Question is: The Element Essential for Gas Exchange
and the answer is: OXYGEN
Word Meaning:
OXYGEN — The chemical element (symbol O) with an atomic number of 8 and relative atomic mass of 15.9994. It is a colorless and odorless gas. Sometimes called elemental oxygen to distinguish it from molecular oxygen.
Definitions from Wiktionary.