Scientists Observed That Over the Past 40 Years,marmots,a Type of Ground

Scientists observed that over the past 40 years,marmots,a type of ground squirrel,are located at increasing elevations on mountains.What is the best explanation for this observation?

A) The marmots are adapting to the cold temperatures.
B) Other species are competing with the marmots for their normal habitat.
C) More snow and rain is falling at the top of the mountains.
D) It is becoming colder at the top of the mountains.
E) It is becoming warmer at the top of the mountains.