The problem is 18 when airplanes fly at higher altitudes, where the difference between the inside and outside pressure increases, causing added stress.
答案：(D) intensified
intensify 是「加劇、增強」，本句指飛機飛得愈高、內外壓力差愈大，問題就愈嚴重；disguised（被掩飾）、understood（被理解）、confronted（面對，被動須用 be confronted with）語意都不合。
你認得形容詞 intense（強烈的）與名詞 intensity（強度），卻很少把動詞 intensify（加劇、增強）記起來，這裡用的又是被動 is intensified，字形一變就不容易認出。
空格前為 is、後接 when 引導的副詞子句，可知此處為被動語態，主詞 the problem 是承受動作的一方。作答線索在同一句後半 where the difference between the inside and outside pressure increases, causing added stress（機艙內外的壓力差增大，因而產生額外的應力），increases 與 added stress 兩項訊息都指向程度升高，故空格應填表示「加劇、增強」的過去分詞，即 intensified。凡是被動語態且四個選項均為過去分詞的題目，判斷步驟相同：先確認主詞是動作的承受者，再從同句或鄰句的訊息判定語意的方向。
confronted 的方向與本句相反：be confronted with 的主詞須是面對問題的人，如 Engineers were confronted with the problem.（工程師面臨這個問題）；本句主詞是 the problem 本身，故不能選。disguised 是「被掩飾」、understood 是「被理解」，與後半句壓力差增大、應力增加的訊息不合。
the problem is intensified（問題加劇）、intensify one's efforts（加緊努力）、intense competition（激烈的競爭）、intensity（強度）、intensive care（加護病房）、at high altitudes（在高空）。
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.