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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i got it wrong

Answers :B,E

What about unconventional - unsurprising?
Since the wars have always brought unforeseeable results, and the point taken by the author is an old one, isn't the idea that wars bring forth unexpected outcomes conventional?

The hint is unforeseeable results: only astounding and stunning make sense according to the sentence.

While reading this sentence most of us instinctively feel like choosing the words ruinous and devastating. These are words that are often used when speaking about war, and for good reason. War IS ruinous and devastating. However, these two words are trick options.

The key to this question lies in the phrase "they produce unforeseeable results.", we all know war is ruinous and devastating, this wouldn't be in line with the 'unforeseeable results' that the sentence is referring to. The only two other word pairs are (unsurprising, conventional) and (astounding, stunning). Of these two choices the second word pair makes the most sense. The first option is directly eliminated since the sentence says that war produces unforeseeable results which is the exact opposite of unsurprising and conventional.

Therefore we choose (B) and (E)