Answer
The main idea of this passage is that in the year 2186, humankind is living underground during a nuclear winter without any government or widespread communication, and is barely managing to subsist in isolated pockets. Therefore, option C and D are incorrect. Option A is partially correct, but it does not address the main idea of the passage, which is the struggle for survival in a post-apocalyptic world. Option B is the best choice as it accurately summarizes the main idea of the passage that in a nuclear conflict, no side can truly win and the consequences can be devastating for all.