The Underlying Cause of the Rissa,Norway,landslide Was ______________

The underlying cause of the Rissa,Norway,landslide was ______________.

A) dissolution of salt in the soil
B) too much gypsum in the soil
C) over-steepening of slopes of volcanic ash
D) freezing and thawing of piles of glacial boulders
E) layering of volcanic pyroclasts