Scientists claim that air pollution causes a decline in the world's average air temperatur

e. 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, the Dutch East Indies, in April 1815. The largest recorded volcanic eruption, Tambora threw 150 million tons of fine ash into the stratosphere. The ash from a volcano spreads worldwide in a few days and remains in the air for years, It’s effect is to mm incoming solar radiation into space and thus cool the earth. For example, records of weather in England show that between April and November 1815, the average temperature had fallen 4.50 ℃. During the next twenty-four months, England suffered one of the coldest periods of its history. Farmers' records from April 1815 to December 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 causes 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 increasingly colder weather.

According to the passage, the effects of Tambora's eruption were______.

A．of several days' duration worldwide

B．felt mainly in the Dutch East Indies

C．evident in the decreased world temperature

D．immediately evident to the world's scientists