Since the 1990s, 30 , all that has changed.
答案：(C) however
語意轉折：「然而自從1990年代開始，這一切都改變了」，however（然而）符合這種與前文封閉歷史的對比轉折；afterwards（之後）、therefore（因此）、furthermore（此外）都不合。
however 你一定認得，但腦中的畫面多半是它出現在句首、後面跟一個逗號。這裡它被夾在句子中間，前後各一個逗號，位置一陌生就讓人懷疑是不是不能這樣用。其實 however 這一類詞歸在連接副詞（linking adjunct）底下，可以出現在副詞能出現的任何位置——句首、句中、句尾都行，只要用逗號隔開（劍橋 English Grammar Today）。
這一格的判斷依據不在這一句裡面，一定要往回讀一句。前一句：Prague was seldom visited by foreigners（布拉格鮮少有外國人造訪）；這一句：all that has changed（那一切都改變了）；再下一句：Prague is now one of the most popular tourist attractions in Europe（布拉格如今是歐洲最熱門的觀光景點之一）。前面是門可羅雀，後面是遊客如織，這是徹頭徹尾的對比，只有轉折語接得上。驗算方法：把兩句用 but 接起來看通不通——Prague was seldom visited by foreigners, but since the 1990s all that has changed. 接得起來，就填 however。101 年第 18 題與 102 年第 17 題也考轉承語插在句子中間，105 年第 16 題則是同一個 however 出現在句首，可以一起對照，把「位置會變、功能不變」這件事記牢。
afterwards（之後）：是時間副詞，只交代先後，不表達對比；而且句首已經有 Since the 1990s 標好時間，再加一個 afterwards 是重複。therefore（因此）：順接因果，填進去會變成「因為鐵幕時期沒人來，所以一切都改變了」，因果對不上。furthermore（此外）：用來追加同方向的訊息，但這一句與前一句方向相反，不是追加。
however（然而，可置於句首、句中或句尾）、since the 1990s（自 1990 年代以來）、all that has changed（那一切都已改變）、a tourist attraction（觀光景點）、one of the most popular（最受歡迎的……之一）、postwar redevelopment（戰後重建）。
Prague, the capital of the Czech Republic, is a very beautiful city. Situated on both banks of the winding River Vltava, Prague is like one big open-air museum. 26 some six hundred years of architecture nearly untouched by natural disaster or war, the city retains much of its medieval appearance. 27 you go, there are buildings in Romanic, Baroque, and Rococo styles that were popular hundreds of years ago. All of them successfully 28 the destruction of postwar redevelopment and remained unchanged.
While the Iron Curtain was still in place under the communist government, Prague was 29 visited by foreigners. Since the 1990s, 30 , all that has changed. Prague is now one of the most popular tourist attractions in Europe.