26 its strength, steel soon became a useful building material.
答案：(B) Because of
Because of its strength（因為它的強度）符合因果邏輯——鋼因為強度高，所以成為實用的建材。Prior to（在……之前）、In spite of（儘管）、Not to mention（更不用說）都不符合這裡的因果語意。
四個選項都是幾個字合起來當一個介系詞用的組合，你會憑中文挑一個，而 In spite of 最眼熟，很容易先被選走。
鑑別度高達 63。先看空格後面接的是什麼：its strength（它的強度），一個名詞組，不是子句——所以四個選項在形態上都過關，得靠邏輯定。前後兩件事是：鋼很堅固、鋼很快成為好用的建材。堅固是原因、成為好建材是結果，方向相同，所以用 Because of（因為）。填進去讀成 Because of its strength, steel soon became a useful building material。順帶記一個對照：because 是連接詞，後面接完整的子句（because it is strong）；because of 是介系詞，後面接名詞（because of its strength）。意思一樣，接什麼決定用哪一個——先看空格後面是名詞還是子句，這一步在克漏字裡幾乎每次都用得上。
In spite of（儘管）要求前後語意相反，但堅固與成為好建材是同一個方向；Prior to（在……之前）講的是時間先後；Not to mention（更別提）用在追加一件更誇張的事，這一句沒有那個語氣。
because of ＋ 名詞（因為）、because ＋ 子句（因為）、due to ＋ 名詞（由於）、in spite of ＋ 名詞（儘管）、prior to ＋ 名詞（在……之前）、not to mention（更別提）。
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.