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。
答對率 20%。
at its best 這個片語課本少見，很容易被 in the least 吸引——因為 not in the least（一點也不）在否定句的考題裡出現得很頻繁，看起來很眼熟。但本句是肯定句，in the least 用不上。
at its best 的意思是「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。本句主詞是極光（this discharge of light），所以用 at its best——「極光最壯觀的時候是在…一帶看到的」。
兩者只差一個 its，意思卻幾乎相反。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）。考試很愛拿這兩個互相干擾，一定要連著記，還可以再加上 at worst（往壞處說）。
前一句 According to scientific measurements, this discharge of light extends from 50 to 100 miles above the earth.（依科學測量，這種光的放電現象出現在地表上方五十到一百英里處）是客觀描述；本句接著點出「在哪些地方看得最清楚」，語氣是正面的推薦，符合 at its best。
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）。
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.