These materials, 24 , were undreamed of in the 18th century.
答案：(B) however
插入語 however 表示語意上的轉折對比（這些材料在現代很普遍，然而在18世紀卻是想像不到的），符合「今昔對比」的語境；finally（最後）、in addition（此外）、as a result（因此）都不符合這種轉折對比的語氣。
however 這個字，你的印象大概只有「出現在句首」那一種擺法，看到它夾在主詞與動詞中間、前後各一個逗號，反而不敢選。
鑑別度高達 61。這是邏輯勾環，線索一定要跨句才看得到。前一句說二十世紀有些椅子是用鋼和塑膠做的；空格這一句 These materials, ______, were undreamed of in the 18th century（這些材料在十八世紀是想都想不到的）把時間往回拉——現在有、過去連想都沒想過，兩句站在相反的位置，所以用 however。填進去讀成 These materials, however, were undreamed of in the 18th century。位置的部分：however 屬於連接副詞，可以出現在副詞能出現的任何位置，夾在主詞與動詞之間、前後各加一個逗號是常見的擺法。指考 97 年第 16 題也考同一個位置的 however，兩題可以擺在一起看。
finally（最後）要求前面有一個順序；in addition（此外）是往同一個方向加東西，但這裡是轉折；as a result（結果）要求前面是原因，但十八世紀想不到這些材料，並不是二十世紀用鋼和塑膠造成的。
however（然而，連接副詞，位置靈活）、nevertheless（儘管如此）、be undreamed of（想都想不到）、as a result（結果、因此）、in addition（此外）、make something possible（使某事成為可能）。
Science makes possible the use of new materials and new methods of producing objects. For example, some 20th-century chairs are made of steel and plastic. These materials, 24, were undreamed of in the 18th century. As new materials develop, one invention often 25 another.
Steel, for instance, was developed by engineers in the 19th century. 26 its strength, steel soon became a useful building material. 27 steel construction, buildings could then have a great many stories. But no one could be expected to walk up 8, 10, or 30 flights of 28.
Therefore, to make tall buildings more accessible to their users, the elevator 29. By providing much-needed space in a world 30 people, tall buildings have solved a great problem of the city and have completely changed our way of life.