Explanation

The colon after the blank indicates a definitional relationship between the blanked word and the phrase that follows the colon. The two answer choices for which “they produce unforeseeable results” would most clearly serve as a definition are “astounding” and “stunning.” While “ruinous” and “devastating” might be adjectives describing the effects of war, they clearly do not fit the logical structure of this sentence, since they are not by definition “unforeseeable.”

Thus, the correct answer is astounding (Choice B) and stunning (Choice E).
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Answers :B,E