Mount St. Helens and the other Cascade volcanoes are ________.
A) a row of young, active, shield volcanoes built as western North America moved over a hot spot deep in the mantle
B) old, deeply eroded, basaltic shield volcanoes built when western North America was over the present-day site of the Hawaiian hot spot
C) old, deeply eroded stratovolcanoes built before the Pacific Ocean existed
D) young, active stratovolcanoes built on a continental margin above a sinking slab of oceanic lithosphere