https://levelread.com/news/level-3/why-airplanes-leave-white-lines-in-the-sky
White lines often (1)______ planes across the sky. These (2)______ contrails(凝结尾 ), which form when planes fly through cold, wet air, (3)______ ice crystals(冰晶) around small particles(颗粒) from the engines. These thin clouds can hold heat in the air and (4)______ warm the planet.
Not all contrails act the same way. Some disappear within minutes, (5)______ others last for hours and spread over large areas. Scientists believe these longer contrails may change the climate, but they are not sure how strong this effect is. So they want to find a way to cut down the problem.
To find out, the UK and Google have started (6)______ 30-month study called Operation Blue Skies. Over the next two winters, air traffic workers (7)______ some flights over the Atlantic. The flights will fly at a different height, up (8)______ 2,000 feet higher or lower. This helps them avoid the areas where contrails are most likely to form. The study is designed (9)______ the effect of these changes. About 10,000 flights will pass through the test area, and hundreds will follow the new rules. In the end, the study wants to learn (10)______ these small changes can really reduce contrails and help cool the planet.
1. A.follow B.follows C.followed
2. A.call B.are called C.called
3. A.make B.making C.to make
4. A.may B.must C.would
5. A.because B.unless C.while
6. A.the B.an C.a
7. A.will ask B.ask C.asked
8. A.to B.at C.from
9. A.testing B.tested C.to test
10. A.that B.whether C.what
词汇短语精讲
1. find a way to 找到……的方法
例句：So they want to find a way to cut down the problem.
提示：find a way to do sth. 为高频结构，表示找到做某事的方法
2. cut down 削减；减少
例句：So they want to find a way to cut down the problem.
提示：cut down 意为削减、减少，可接 problem/costs/pollution 等宾语
3. act the same way 表现相同
例句：Not all contrails act the same way.
提示：act 在此意为“表现、起作用”，act the same way 表示以相同方式表现
4. be designed to do 被设计用来做……
例句：The study is designed to test the effect of these changes.
提示：be designed to do sth. 为固定结构，主语通常是物，表示用途或目的
5. most likely to 最可能……
例句：This helps them avoid the areas where contrails are most likely to form.
提示：be likely to do 意为“很可能做某事”，most likely 表最高程度
句子精讲
1. Some disappear within minutes, while others last for hours and spread over large areas.
结构：分句一（Some disappear within minutes）+ while 连接 + 分句二（others last for hours and spread over large areas）
语法：while 连接两个并列分句表示对比；分句二内 and 连接两个并列谓语 last 与 spread
仿写：可用句式：Some do A, while others do B. 用于对比两类事物的不同表现。
2. Scientists believe these longer contrails may change the climate, but they are not sure how strong this effect is.
结构：主句（Scientists believe + 宾语从句）+ but 转折 + 主句（they are not sure + how 引导的宾语从句）
语法：believe 后接 that 省略的宾语从句；be not sure how 后接宾语从句表“不确定……程度”
仿写：可用句式：X believe(s) (that)…, but … is/are not sure how…. 用于表达“有观点但又不确定”。
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