<綜合測驗>第16至20題為題組 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 ______ when airplanes fly at higher altitudes， where the difference between the inside and outside pressure increases， causing added stress. (A) disguised (B) understood (C) confronted (D) intensified
