The only thing certain is that the Nazca lines remain one of the world's most fascinating 30 mysteries.
答案：(D) unsolved
unsolved mysteries（未解之謎）為固定搭配，表示納斯卡線至今仍是世界上最引人入勝的未解謎團；unchanged／undetermined／unfolded 均不符合 mysteries 的固定搭配語意。
四個選項都是 un- 開頭的形容詞，長得很像，容易混淆，你會憑中文挑一個。
答對率四成三。先看整句：The only thing certain is that the Nazca lines remain one of the world's most fascinating ______ mysteries（唯一確定的是，納斯卡線仍是世上最迷人的______之謎）。關鍵在同一句的 The only thing certain is（唯一確定的只有這件事），言下之意是其他都還沒有答案。前面幾句也把各種猜測列了一遍：有人說是外星人的降落場、有人說是引水道、有人說是日曆，全都只是「有人說」。所以這些謎至今沒有被解開，填 unsolved。填進去讀成 one of the world's most fascinating unsolved mysteries（世上最迷人的未解之謎之一）。unsolved mysteries 是成組的說法，整組記起來就好。
unchanged（未改變的）講的是狀態沒變，但謎的重點不在變不變；undetermined（未確定的）語意雖然接近，但它通常用在數量、日期、結果這類還沒定案的事情上（The date is still undetermined.），不與 mystery 搭配；unfolded 是 unfold（展開、逐漸揭露）的過去分詞，意思會變成「已經展開的」，方向剛好相反。三個都是 un- 開頭的形容詞，形態吻合，容易被順手選走。
unsolved mysteries（未解之謎）、solve a mystery（解開謎團）、unfold（展開、逐漸揭露）、undetermined（未確定的）、remain ＋ 補語（仍然是……）、fascinating（迷人的）。
In the desert of southwest Peru, enormous shapes, complex patterns, and thousands of perfectly straight lines are cut into the desert's surface. They are known as the Nazca lines and they 26 500 square kilometers. There are about fifty animal figures 27 275 meters long, including a monkey, a spider, and an "owl man." The last of these lines were drawn about 1,000 years ago.
More 28 is the fact that they can be viewed only from the air. Some people believe the Nazca Indians were 29 able to fly, perhaps in balloons. Others say the lines were landing areas for alien spaceships.
Maybe the lines were to carry water for farming, or were used as a calendar. The only thing certain is that the Nazca lines remain one of the world's most fascinating 30 mysteries.