In the early days of aviation, plane windows 17 square in shape.
答案：(A) used to be
【句型】used to be 字詞搭配「過去曾經是」，表示「早期飛機窗戶『曾經』是方形的」（現在不是了）；were to be（表計畫/命運）、would have been（過去完成條件式）、must have been（過去推測，這裡是明確的歷史事實不是推測）都不合。
used to 很容易和另外兩個長得像的說法混在一起。一次分清楚：used to ＋ 原形動詞 ＝ 過去曾經（現在不是了）；be／get used to ＋ 名詞或動名詞 ＝ 習慣於；be used to ＋ 原形動詞 ＝ 被用來做……。 本題是第一個。
句子是 In the early days of aviation, plane windows ___ square in shape（在航空業早期，飛機窗戶 ___ 方形的）。時間副詞 In the early days 已經定調是過去；而文章第一句就說 all plane windows have rounded edges（現在所有飛機窗戶都是圓角）。過去是方的、現在是圓的，這種今昔對比正是 used to 的核心語意——它不只講過去，還暗示「現在已經不是了」。
were to be 表示「按計畫或註定要……」，放進來變成「窗戶註定要是方的」，語意不通；would have been 是與過去事實相反的假設；must have been 是對過去的推測（一定是……吧），但方形窗是文章明確陳述的歷史事實，不是猜測。
used to V（過去曾經……，現在不是了）、be／get used to Ving（習慣於做……）、be used to V（被用來做……）、in the early days of（在……的早期）、be square／round in shape（形狀是方的／圓的）。
Airline passengers may have noticed that all plane windows have rounded edges, instead of the hard corners commonly found in our house. The round windows are indeed pleasant to the eye, but they actually were designed for reasons 16 aesthetics.
In the early days of aviation, plane windows 17 square in shape. Then as commercial air travel became popular in the 1950s and airplanes began flying higher and faster, three planes mysteriously broke apart in midair. The cause? Square windows. Scientists found that sharp corners are natural weak spots where stress concentrates. The problem is 18 when airplanes fly at higher altitudes, where the difference between the inside and outside pressure increases, causing added stress. When subjected to repeated pressurization high in the sky, the four corners on a square window may 19 .
Curved windows, on the other hand, distribute stress around more evenly, reducing the likelihood of cracks or breaks. Circular shapes are also stronger and resist deformation, and therefore can 20 extreme differences in pressure inside and outside of an aircraft. Thus, round windows are a major safety innovation that keeps planes from disintegrating mid-flight. They are also used on ships and spacecraft for their greater structural integrity.