Science is still not certain 16 exactly what these lights are and what causes them.
答案：(C) as to
【句型】certain as to + 問題，as to 表示「關於」，not certain as to exactly what...是固定用法，表示「對於……究竟是什麼並不確定」。due to（由於）、instead of（而不是）、in spite of（儘管）都不符合這裡的搭配。
四個選項都是幾個字合起來當一個介系詞用的組合，你會憑哪個中文順挑一個，其中 due to 最眼熟，很容易先被選走。
答對率四成。先看空格後面接的是什麼：exactly what these lights are and what causes them——兩個 what 引導的名詞子句。所以空格要的是一個能帶出「關於什麼」的說法。四個選項裡只有 as to 是「關於、至於」，而且它後面正好可以接疑問詞引導的名詞子句（as to what、as to whether）。前面的 not certain 與它成組：be certain 或 uncertain as to ＋ 疑問詞子句，就是「對……確定或不確定」。填進去讀成 Science is still not certain as to exactly what these lights are（科學界對這些光究竟是什麼仍不確定）。
due to（由於）帶出的是原因；instead of（而不是）帶出的是被取代的那樣東西；in spite of（儘管）要求前後語意相反。三個後面都只接名詞或動名詞，接不上這裡的 what 子句，語意也都不是「關於」。
as to ＋ 疑問詞子句（關於、至於）、be uncertain as to whether（不確定是否）、due to（由於）、instead of（而不是）、a dazzling spectacle（令人目眩的奇景）、the northern lights（北極光）。
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.