But when they thought they were alone (though they 18 by a secret camera), their faces twisted into vivid mixes of anguished distress, fear, and disgust.
答案：(A) were being taped
過去進行式被動語態 were being taped，表示「當時正被祕密攝影機拍攝著」，與主要子句過去式 thought 的時態一致，且學生是「被拍攝」的對象（被動關係）。had taped／are taping 都是主動語態（語意錯誤，學生不是拍攝者），have been taped 是現在完成式被動，時態不符合過去語境。
四個選項都是 tape 的各種時態與語態組合排在一起，你會想一次判完時態、語態兩件事，很容易只顧到一邊。
鑑別度高達 55，分開的是有沒有把兩件事分開判、而且兩件都判對。先判語態：主詞是 they（那些學子），學子是「被祕密攝影機拍攝」的一方，不是拍攝的人，所以必須是被動，had taped 與 are taping 都是主動，先出局。再判時態：括號外的主要動詞是 thought（過去式），括號裡講的是他們自以為獨處的「那個當下」正在發生的事，所以要用過去進行式的被動 were being taped（當時正被拍攝著）。填進去讀成 though they were being taped by a secret camera（其實當時正被祕密攝影機拍著）。
have been taped 是現在完成式的被動，它的時間落在「到現在為止」，與整段的過去時間軸接不上；had taped 若讀成過去完成式，方向是往更早推，但這裡要講的是同一個當下正在進行的事，不是更早發生的事。
was／were being ＋ 過去分詞（過去進行式的被動）、tape（動詞：錄影、錄音）、twist into（扭曲成）、though ＋ 子句（雖然、其實）、a secret camera（祕密攝影機）、anguished distress（極度的痛苦不安）。
People may express their feelings differently on different occasions. Cultures sometimes vary greatly in this regard.
A group of researchers in Japan, 16, studied the facial reactions of students to a horror film. When the Japanese students watched the film 17 the teacher present, their faces showed only the slightest hints of reaction. But when they thought they were alone (though they 18 by a secret camera), their faces twisted into vivid mixes of anguished distress, fear, and disgust.
The study also shows that there are several unspoken rules about how feelings should be 19 shown on different occasions. One of the most common rules is minimizing the show of emotion. This is the Japanese norm for feelings of distress 20 someone in authority, which explains why the students masked their upset with a poker face in the experiment.