[单项选择]For the last hundred years the climate has been growing much warmer. This has had a number of different effects. Since the beginning of the 20th century, glaciers (冰河) have been melting very rapidly. For example, the Muir Glacier in. Alaska has retreated 2 miles in 10 years. Secondly, rising temperatures have been causing the snowline to retreat on mountains all over the world. In Peru, for example, it has risen as much as 2,700 feet in 60 years.
As a result of this, vegetation has also been changing. In Canada, the agricultural crop line has shifted 50 to 100 miles northward. In the same way cool-climate trees like birches and spruce have been dying over large areas of Eastern Canada. In Sweden the tree line has moved up the mountains by as much as 65 feet since 1930.
According to the passage, the Muir Glacier has retreated because ______.
A. glaciers have been melting very gradually
B. the climate has been growing much warmer
C. the snowline has retreated on mountains
D. vegetation has been changing