A world 17 good manners would be confusing and potentially confrontational.
答案：(D) without
a world without good manners（一個沒有良好禮儀的世界）表示假如世界缺乏良好禮儀，將會令人困惑甚至引發衝突；about／through／under 均不符合此處語意。
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四個都是介系詞，後面都接得上 good manners，一個一個讀好像都不算太怪，很容易憑感覺挑。
關鍵在動詞的形態：would be。句子裡沒有 if 子句卻用了 would，通常代表條件被藏進主詞裡了——A world ______ good manners 這一整包名詞組就是那個條件，等於 If there were no good manners in the world, it would be ...。這是課本教的與現在事實相反的假設語氣：if 子句用過去式、主要子句用 would ＋ 原形。既然與事實相反，而現實中是有禮節的，那個假設就只能是「沒有禮節的世界」，所以填 without。後半句的預測（會令人困惑、可能起衝突）也只有從「沒有禮節」才推得出來。
about（關於）會變成「一個關於禮節的世界」，不成句。through（穿過、藉由）接不起 world 與 manners 的關係。under 雖然可以說 under the rules（在規則之下），但那樣禮節是存在的，推不出「令人困惑、可能起衝突」，也與 would be 的假設語氣對不上。
without ＋ 名詞（沒有……）、if 子句用過去式配主要子句 would ＋ 原形（與現在事實相反的假設）、under the rules（在規則之下）、confrontational（好起衝突的、對立的）、potentially（可能地）、confusing（令人困惑的）
Having good manners is more than just being nice. Good manners make the social world easier to navigate. They provide guidelines to 16 in times of uncertainty and reduce the chance of awkward exchanges. For example, knowing who is supposed to go first or have a seat keeps us from having to negotiate every time. A world 17 good manners would be confusing and potentially confrontational.
Good manners certainly connect people to their 18 environment. Respecting good manners shows a desire to actively engage with a community and the people in it. 19 to adopt good manners sets people apart.
We might forgive people who let their manners slip, but we'll feel more positive about and strongly connected to those who 20. Good manners are very important because they teach us the proper way to behave in society.