Today, there are over 420 submarine cables 16 , stretching over 700,000 miles around the world.
答案：(B) in service
【慣用語】in service 意指「正在使用中／運作中」，符合「超過420條海底電纜正在運作」的文意。at large（大體上／逍遙法外）、by contrast（相較之下）、under control（受到控制）皆不符文意。
四個選項都是「介系詞加名詞」的片語，都可以掛在名詞後面，讀起來都像是在補充說明那 420 條電纜。如果對這幾個片語只有模糊的印象，這一題就變成猜。
先問空格補充的是什麼。there are over 420 submarine cables ___ 講的是現在有超過 420 條海底電纜處於某種狀態，所以空格要的是一個表狀態的片語。in service 就是「使用中、運作中」（劍橋詞典 in service：being used），它有一個天天看得到的對照組，就是公車與捷運車廂上的 Out of Service（暫停服務）。把 in service 擺回去讀一次：如今全世界有超過 420 條海底電纜正在運作，總長超過 70 萬英里，語意完全順。鑑別度 43，分的是認不認得這一組片語，本題不必回頭讀上下文。
at large 有兩個常見意思，the public at large 是「一般大眾」，逃犯 at large 是「在逃」，兩個都掛不到電纜身上。by contrast（相較之下）是轉承語，要用逗號隔開放在句首或句中，用來對比兩件事，這裡既沒有對比的對象，它也不能直接當名詞的修飾語。under control（在控制之下）講的是局面沒有失控（The situation is under control.），電纜不是被控制住的東西。
in service（使用中、運作中）、out of service（暫停服務、故障中）、at large（一般大眾；在逃）、under control（受到控制）、stretch over（綿延、長達）、a submarine cable（海底電纜）
When we stream the latest TV series, or download high-resolution photos, we are probably unaware that the data behind them is speeding around the world in cables under the sea.
These cable systems, faster and cheaper than satellites, carry most of the intercontinental Internet traffic. Today, there are over 420 submarine cables 16 , stretching over 700,000 miles around the world. It is not a new phenomenon, 17 . The first transcontinental cable—laid in 1854—ran from Ireland to Newfoundland, and made telegraph communication possible between England and Canada. Currently, the world's highest-capacity undersea Internet cable is a 5,600-mile link between the US and Japan. 18 named "FASTER," the cable connects Oregon in the US with Japan and Taiwan.
The submarine cables require extra 19 to install. They must generally be run across flat surfaces of the ocean floor, and stay clear of coral reefs, sunken ships, fish beds, and other general 20 . The fiber-optic cables are also very fragile, so they are surrounded with layers of tubing and steel to prevent damage.