According to scientific measurements, this discharge of light 19 from 50 to 100 miles above the earth.
答案：(A) takes place
描述科學事實／現象的恆常狀態，用簡單現在式 takes place。was taking place（過去進行式）、had taken place（過去完成式）、took place（簡單過去式）都是描述過去特定事件，不符合這裡陳述科學事實的語境。
四個選項是同一個片語的四種時態，你會去找句子裡有沒有時間副詞，找不到就憑語感挑一個。
答對率四成三，鑑別度高達 54。先確認這一句在做什麼事：According to scientific measurements（根據科學測量），後面講的是北極光發生在離地五十到一百英里的高度——這是一個一直都成立的事實，不是某一次發生過的事件。陳述科學事實、自然現象與恆常狀態，一律用簡單現在式，所以填 takes place。填進去讀成 this discharge of light takes place from 50 to 100 miles above the earth。整篇的動詞形態也能拿來驗算：前後的 is、spread、looks、hang、sway 全是現在式，這一格跟著它們對齊就好。
took place 是簡單過去式、was taking place 是過去進行式、had taken place 是過去完成式，三個都把這件事鎖在過去某個時間點上，與「科學測量得出的恆常事實」對不上，而且句子裡沒有任何過去的時間標記可以搭。
take place（發生、舉行）、according to（根據）、scientific measurements（科學測量）、discharge（放電、排放）、above the earth（在地球上方）、from A to B（從 A 到 B 的範圍）。
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.