Therefore, to make tall buildings more accessible to their users, the elevator 29 .
答案：(C) was invented
electric elevator 是「被發明」的對象（被動關係），且敘述 historical past 事件用簡單過去式被動語態 was invented；invented（主動語態，語意錯誤——電梯不會主動發明）、had invented／would have invented 都是主動語態，也不符合這裡單純敘述過去發生的事實。
四個選項都是 invent 的不同形態，你會先想時態，但這一題真正要先判的是語態。
鑑別度高達 55。先判語態：主詞是 the elevator（電梯），電梯不會自己發明東西，它是被人發明出來的，所以必須用被動。四個選項裡只有 was invented 是被動——這一步就定了。再看時態：整段講的是十九世紀的事，配過去式沒有問題。填進去讀成 to make tall buildings more accessible to their users, the elevator was invented（為了讓高樓更方便進出，電梯就被發明出來了）。判主動被動最快的一招就是問一句：主詞是做這個動作的那一方，還是被做的那一方。
invented 與 had invented 都是主動，會讀成「電梯發明了別的東西」；would have invented 是與過去事實相反的假設，但這一句陳述的是真實發生過的事，不是假設。
be invented（被發明）、被動語態（主詞是承受動作的那一方）、make A more accessible to B（使 A 對 B 更容易進出或取得）、an elevator（電梯）、solve a problem（解決問題）、way of life（生活方式）。
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.