The author mentions the eruption of Mount St. Helens (lines 25 - 26) primarily in order to emphasize which of the following points

A． The deep and enduring effect that volcanic eruptions have on the environment
B． The process by which classic volcanic eruptions tend to occur
C． The relative dearth of cases with scientists can judge the effects of volcanic eruptions
D． A sample case with which later volcanic explosions are compared
E． The global cooling effects of one of the best-studied volcanic eruptions