Farther north, the aurora frequently looks like fiery draperies which hang from the sky and sway 18 while flames of red, orange, green, and blue play up and down the moving folds.
答案：(C) to and fro
to and fro 意為「來回地、前後擺盪」，形容極光如布幔般在天空搖曳；here and now（此時此地）、by and large（大體而言）、more and more（越來越）語意都不合。
答對率 39%。
四個選項都是「A and B」形式的固定片語，字面拆開都看得懂，合起來卻各有各的意思——這類片語只能整組記。to and fro ＝ 來回地（fro 是古英文的 from，現在只保留在這個片語裡）：The swing moved gently to and fro.（鞦韆輕輕地來回擺盪）。
空格前的動詞是 sway（搖曳），後半句又說 flames… play up and down the moving folds（火焰在飄動的褶皺間上下跳躍）——「up and down 與 to and fro 一個上下、一個左右，是刻意並列的對仗」，看到後半句的 up and down，前半就該想到同類型的方向片語。
to and fro（來回）、up and down（上下）、back and forth（來回，同 to and fro）、here and there（到處）、by and large（大體而言）、here and now（此時此地）。
sway to and fro（來回擺盪）、back and forth（來回）、up and down（上下）、here and there（到處）、by and large（大體而言）。
The northern lights, known as the aurora borealis, is one of nature's most dazzling spectacles. Science is still not certain 16 exactly what these lights are and what causes them.
Sometimes, the brilliant rays of light spread upward in the shape of a fan. 17, they flash here and there like giant searchlights. Farther north, the aurora frequently looks like fiery draperies which hang from the sky and sway 18 while flames of red, orange, green, and blue play up and down the moving folds.
According to scientific measurements, this discharge of light 19 from 50 to 100 miles above the earth. It is seen 20 around the Hudson Bay region in Canada, in northern Scotland, and in southern Norway and Sweden.