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

[EX Q]: Passage: Cloud types include cirrus, stratus, and cumulus. Answer: clouds
[EX A]: Cirrus, stratus, and cumulus are types of what?

[EX Q]: Passage: Substances may be either elements or pure compounds. They may also be solid, liquid, or gas. Some of the elements (such as silver) exist in the solid form. Other elements (mercury) exist as liquids at room temperature. Elements such as helium and radon exist as gases at room temperature. A compound such as water will take the form of a solid, a liquid, or a gas, depending upon the temperature. Answer: pure compounds
[EX A]: Substances may be either elements or what?

[EX Q]: Passage: The job of the respiratory system is the exchange of gases between the body and the outside air. This process, called respiration , actually consists of two parts. In the first part, oxygen in the air is drawn into the body and carbon dioxide is released from the body through the respiratory tract. In the second part, the circulatory system delivers the oxygen to body cells and picks up carbon dioxide from the cells in return. The lungs are organs of the respiratory system. It is in the lungs where oxygen is transferred from the respiratory system to the circulatory system. Answer: gases
[EX A]:
What does the respiratory system exchange between the body and the outside air?