Definition: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Gas exchange is needed to provide cells with the oxygen they need for cellular respiration. Cells cannot survive for long without oxygen. Gas exchange is also needed to carry away carbon dioxide waste. Some of the carbon dioxide in the blood dissolves to form carbonic acid, which keeps blood pH within a normal range. Answer: gas exchange
Output:
What is needed to provide cells with the oxygen they need for cellular respiration?