Based on the information provided, the following statements are true regarding Washington's landscape:
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The scablands in the Columbia Plateau were formed from massive erosion due to floods. - This statement is true. The scablands were created by glacial flood events, particularly the Missoula Floods.
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At one point, the Pacific Northwest was covered by glaciers, which have since melted. - This statement is also true. The region was affected by glaciation during the last Ice Age, leading to significant geological features formed by glacial activity.
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Mt. Rainier is considered the most dangerous volcano in North America. - This statement is typically regarded as true due to its potential for explosive eruptions and the population density surrounding it.
The first statement regarding frequent volcanic eruptions in the Cascade Mountains needs more clarification, as while the region is volcanically active, "frequent" can be subjective depending on the time frame considered. However, it can be said that there have been eruptions in the recent past.
Thus, the selections that are distinctly true are the second, third, and fourth statements.