It is not a new phenomenon, 17 .
答案：(D) though
【轉折語】though 置於句尾，表示「不過／然而」，用來補充轉折語氣，呼應下文說明海底電纜其實早在1854年便已出現。then（那麼）、still（仍然）、instead（取而代之）皆不符合此處句尾轉折的用法。
though 這個字，你的印象大概只有連接詞的「雖然」，而且一定要放在句首帶一個子句（Though it was raining, we went out.）。本題它單獨掛在句尾，前面一個逗號、後面就是句號，位置一陌生就會覺得這樣不成句。
先看空格站的位置：It is not a new phenomenon, ___. 空格前是逗號、後是句號，整句只有一套主詞與動詞，所以空格要的是一個副詞。though 除了當從屬連接詞，也可以當副詞，常掛在句尾表示「不過」，用來補上與前面所說相反的一句（劍橋 English Grammar Today 的 though 條目）。口語裡很常聽到：It's expensive. I like it, though.（很貴，不過我喜歡。）語意方向也對得上：前面才說今天有超過 420 條電纜在運作、綿延 70 萬英里，聽起來像是新科技，本句卻說這並不是新現象，正好需要一句表相反的補充。鑑別度 54，分的是知不知道 though 還有副詞這個身分。
then（那麼）也能放句尾（What did you do then?），但它表示的是「既然如此」的推論，前面沒有可以拿來推論的條件。still（仍然）當副詞時通常放在主詞與動詞之間或句首，不掛在句尾的逗號後面，而且它是「仍然」，不表相反。instead（取而代之）確實可以放句尾（I didn't go; I stayed home instead.），所以位置刷不掉它，要靠語意：instead 要求「不是 A 而是 B」的替換，本句沒有被替換掉的東西。
though（掛在句尾當副詞：不過、然而）、even though（雖然，後面接完整子句）、instead（取而代之，可放句尾）、a new phenomenon（新現象）、make sth possible（使…成為可能）、carry Internet traffic（承載網路流量）
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.