Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word for each of the blanks from 36 to 40. Carbon dioxide in the atmosphere, which is primarily a result of mankind's burning of fuels, is thought to act like the glass of a greenhouse. It absorbs heat radiation from the earth and its atmosphere, heats that otherwise would dissipate into space. The possibility that the greenhouse effect could alter the earth's temperature has been debated for ...

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word for each of the blanks from 36 to 40.

Carbon dioxide in the atmosphere, which is primarily a result of mankind's burning of fuels, is thought to act like the glass of a greenhouse. It absorbs heat radiation from the earth and its atmosphere, heats that otherwise would dissipate into space. The possibility that the greenhouse effect could alter the earth's temperature has been debated for many years. Scientists have agreed that carbon dioxide is increasing but there has been uncertainty about whether temperatures are also increasing. The major difficulty in accepting the greenhouse effect has been the absence of observed warming coincident with the historic carbon dioxide increase.

Doubts about the greenhouse effect seem to center on the fact that ______.