级差率=（）×100%。

A.后级运价率/前级运价率

B.前级运价率/后级运价率

C.某级运价率/第一级运价率

D.第一级运价率/某级运价率

A volcanic eruption in Iceland has sent ash across northern Europe. Airlines have stopped or changed the flights across the Atlantic Ocean, leaving hundreds of passengers stuck in airports.
Grimsvotn is one of the largest active volcanoes in Europe. What makes Grimsvotn different is that it lies under a huge glacier(冰川) of ice up to 12 meters thick. The hot volcano heats up the ice above it, which then forms a layer(M) of water between the glacier and the volcano. This layer of water puts pressure on the volcano ,keeping it stable. As the water flows out from under the glacier, the pressure Rifts. The lava(岩浆) from the volcano then comes up to the surface. This is exactly what has happened today.
Now, airlines have to make changes to their flights so as not to fly through the clouds of volcanic ash. According to KLM, one of Europe’s biggest airlines, airplanes cannot go under the cloud or over it. Going through the cloud can result in ash getting stuck in the airplane’s engines, causing damage to the plane.
The eruption has also caused problems for animals in Iceland. The volcano left ash and sharp, glass-like rocks all over the countryside. Farmers are keeping their animals inside to stop them from eating ash-covered grass or the sharp objects.

This text is most probably taken from ______.

A．a research paper

B．a newspaper report

C．a class presentation

D．a geography textbook