Q. Assertion If earth did not have atmosphere, its average surface temperature would be lower than what is now.
Reason Green house effect of the atmosphere would be absent if earth did not have atmosphere.

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Solution:

Earth is heated by sun's infrared radiation. The earth also emits radiation most in infrared region. These radiations are reflected back by heavy gases like is atmosphere. These back radiation keep the earth's surface warm at night. This phenomenon is called green house effect. When the atmosphere were absent then temperature of earth falls.