It can be concluded from information contained in the text that the average temperature at an altitude of I kilometer above the Earth is about ______.

A．discuss the significance of increasing the amount of carbon dioxide in the atmosphere
B．warn of the dangers of continued burning of fossil fuels
C．explain how a constant temperature is maintained in the Earth's surface
D．demonstrate the usefulness of mathematical models in predicting long-range climatic change

A．Objective.
B．Mildly critical.
C．Indifferent.
D．Pessimistic.