Synonyms, the easy-to-use comparing technique, is seen in every text!
Certainty means a firm conviction that something is the case.
I think the correct answer is D. Because, of course, every integration is difficult in the beginning, but as time goes by, it gets easier, and it is the only way both cultures can survive. If they don't succeed in integrating, this will inevitably lead to a war in which many people, if not a whole culture, will be annihilated.
