Which sentence breaks the unity of the paragraph below written by C.D.B. Bryan?
During the later stages of World War II, pilots of advanced fighter aircraft experienced strange and alarming behavior on the part of their machines when in a high-speed dive they approached the speed of sound. Their planes buffeted and wrenched about. Controls sometimes reversed or became ineffective, and increased drag set in. The Concorde regularly flies at several times the speed of sound. In some cases the wings tore off, and the plane broke apart and crashed. Because of these experiences there developed the idea of a sonic wall, a "sound barrier" of some kind through which no aircraft could pass unscathed.
The sentence which breaks the unity of the paragraph is the fourth sentence,
"The Concorde regularly flies at several times the speed of sound."
All the other sentences address the topic introduced in the first sentence of the paragraph, which is the problems encountered when World War II advanced fighter aircraft approached the speed of sound during high-speed dives. There are three sentences which describe exactly how these problems were manifest, with the planes being "buffeted and wrenched about," the controls becoming ineffective and drag setting in, and, in extreme cases, the planes actually breaking apart and crashing. The last sentence of the paragraph ties the previously information together, telling what was done to fix the problem introduced at the beginning of the paragraph and developed thereafter.
Although the sentence about the Concorde talks about one part of the topic, the sound barrier, it does not address the main point of the topic, which is the fighter planes' problems with it. Thus, the sentence about the Concorde breaks up the unity of the paragraph.
In this paragraph the sentence, The Concorde regularly flies at several times the speed of sound, is most obviously out of place. The whole paragraph discusses an issue which is related to World War II which ended in 1945. The writer C.D.B. Bryan discusses a seemingly insurmountable barrier the fighter jets during World War II experienced. When the fighter jets of World War II which could travel only at transonic speeds approached the speed of sound their planes began to experience the effects of what in aerodynamics is called 'compressibility.'
The Concorde however was a supersonic passenger airliner which made its inaugural flight in 1969. By now scientists had solved the problem of 'compressibility' which aircraft experienced when breaking the sound barrier.
The golden rule in paragraph construction is:
1 idea = 1 paragraph
Since the sentence, The Concorde regularly flies at several times the speed of sound, is not related to the idea of the problem which fighter jets of an earlier era experienced it should be deleted from this paragraph.