It is seen 20 around the Hudson Bay region in Canada, in northern Scotland, and in southern Norway and Sweden.
答案：(D) at its best
at its best 意為「處於最佳、最壯觀的狀態」：極光在加拿大哈德遜灣一帶、蘇格蘭北部與挪威、瑞典南部看得最為壯觀；in the least（一點也不）用於否定句，on the whole（整體而言）、for its sake（為了它）都不合。
at its best 這個片語課本少見，四個選項擺在一起時，最眼熟的往往不是答案。
先看句子缺什麼。It is seen ___ around the Hudson Bay region... 主詞 It、動詞 is seen 都已經齊全，空格填的是修飾動詞的副詞片語，四個選項都是介系詞片語，句法一樣過關，要靠語意與搭配決定。先用一個句法線索刷掉一個：in the least（一點也不）是強調用語，只用在否定句與疑問句，標準說法是 not in the least；本句是肯定的直述句，它進不來。剩下的靠語意。at its best 是「處於最佳狀態、最出色的時候」，其中的所有格會隨主詞改變：at his best、at her best、at their best。本句主詞 It 回指前面談的極光，所以用 its——「極光最壯觀的時候是在這幾個地方看到的」。驗算法：把 its 換成它回指的名詞唸一遍——the aurora is seen at the aurora's best around the Hudson Bay region，所有格對得上主詞，方向就對了。前一句是科學測量的客觀敘述（這種光出現在地表上方五十到一百英里處），本句接著點出在哪些地區看得最壯觀，語氣是正面的，與 at its best 相合。順帶把最容易混淆的一組連著記：at its best 與 at best 只差一個 its，意思幾乎相反。at its best ＝ 處於最佳狀態（正面）；at best ＝ 充其量、頂多（帶保留或負面語氣）：The film was at best mediocre.（這部片頂多算普通。）再加上 at worst（往壞處說），三個一起記最不會亂。
in the least 如前所述受句型限制，肯定句用不上。on the whole 是「整體而言」，用來總結全面情況，但本句正好相反，講的是特定幾個地區看得最好，不是整體概況。for its sake 是「為了它」，for the sake of 表示目的或利益，而「被看見」這件事沒有一個為誰而做的目的，語意接不上。
at its／his best（處於最佳狀態）、at best（充其量）、at worst（往壞處說）、not in the least（一點也不）、on the whole（整體而言）、for the sake of（為了…的緣故）。
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.