Answer: B
Correct answer: A rapid mass movement of snow and ice down a slope
An avalanche is the rapid downslope movement of a mass of snow, often carrying ice, rock and debris with it. It is a form of mass wasting, driven by gravity once the strength holding the snowpack in place is exceeded. The commonest and most dangerous type is the slab avalanche, in which a cohesive layer of snow releases across a weak layer beneath it - typically buried surface hoar or faceted crystals formed during a cold clear spell. Slopes between about 30 and 45 degrees are the most prone: gentler ones do not release, steeper ones shed snow continuously. Triggers include fresh heavy snowfall, rapid warming, rain on snow, and the weight of a person crossing the slope, which accounts for most fatal avalanches. They are a serious hazard in the Himalaya, particularly for troops posted at high altitude and along routes such as the road to Siachen, and in Himachal Pradesh, Uttarakhand and Jammu and Kashmir.