【单选题】

Scientists claim that air pollution causes a decline in the world’ s average air temperature. In order to prove that theory, ecologists have turned to historical data in relation to especially huge volcanic eruptions. They suspect that volcanoes effect weather changes that are similar to air pollution.

One source of information is the effect of the eruption of Tambora, a volcano in Sumbawa, theDutchEast Indies ( the former name of the Republic of Indonesia) ,inApril 1815. The largest recorded volcanic eruption, Tambora threw 150 million ton of fine ash into the stratosphere. The ash from a volcano spreads worldwide in a few days and remains in the air for years. Its effect is to turn in coming solar radiation into space and thus cool the earth. For example, records of weather inEng land show that betweenApril and November 1815, the average temperature had fallen 4.5°F during the next twenty-four months,England suffered one of the coldest periods of its history. Farmers’ re cords fromApril 1815 toDecember 1818 indicate frost throughout the spring and summer and sharp decreases in crop and livestock markets. Since there was a time lag of several years between cause and effect, by the time the world agricultural commodity community had deteriorated, no one realized the cause.
Ecologists today warn that we face a twofold menace. The ever-present possibility of volcanic eruptions, such as that of Mt. St. Helens in Washington, added to man’ s pollution of the atmosphere with oil, gas, coal, and other polluting substances, may bring us increasingly colder weather.
No one realized the cause of the deterioration of the world agricultural commodity market because ______.
A、there was a long interval between cause and effect
B、the weather was difficult for us to forecast
C、weather forecasts were inaccurate
D、ecologists didn’t exist until modem times