听力原文：M: Ozone is an extremely active gas comprised of three atoms of oxygen. Ozone exists naturally in the Earth's upper atmosphere, the stratosphere, where it shields the Earth from the sun's ultraviolet rays. However, ozone found close to the Earth's surface, called ground-level ozone, is a component of smog and a harmful substance.
W: What is the ground-level ozone? Where does it come from?
M: Ground-level ozone is a colorless gas that can be found in the air we breathe. It is formed through a complex chemical reaction between volatile organic compounds and nitrogen oxides in the presence of sunlight. It is hard to be watched by eyes.
W: Who is considered most at risk from exposure to ground-level ozone?
M: Children are the most at risk from exposure to ground-level ozone: Their respiratory systems are still developing and they breathe more air per pound of body weight than adults.
W: From what you've just said, can I infer that ground-level ozone is closely related to summer, the hot days?
M: Ground-level ozone is a summer time problem; children spend most of the summer outside playing at summer camps, playgrounds, and neighborhood parks and in backyards.
W: How do I know when ground-level ozone concentrations are high?
M: Ground-level ozone concentrations tend to be especially high during summer when the weather is hot and sunny.
W: Is there any organization providing relevant information?
M: The staff from the departments concerned has developed a procedure to inform. the public about expected ground-level ozone readings. Each afternoon from May 1st through September 15th, the staff retrieve and review monitored ozone data and weather forecast information. Then they develop ground-level ozone forecasts for the following day.
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A．It exists close to the surface of the Earth.
B．It is a component of smog and a harmful substance.
C．It is a kind of yellow gas.
D．It is formed through a complex reaction.