Eighty Years Ago,a Predatory Crab Was Introduced to a Region

Multiple Choice

Eighty years ago,a predatory crab was introduced to a region it previously did not inhabit and began preying on a species of marine snail.A scientist was able to locate museum specimens of the snail's shells from prior to the crab's introduction and to compare their thickness-an important factor in how protective the shells are-to the thickness of shells from current snails.Based on the graphs of her results (shown here) ,what form of evolution/natural selection has taken place?

A) nonadaptive evolution
B) stabilizing selection
C) directional selection
D) diversifying selection
E) neutral selection