Once the questions are finished, the examiner analyzes the results to determine if the person tested 24 truthful.
答案：(A) was being
was being truthful 用 be 動詞的進行式，表示「當時（測謊進行時）是否說實話」這個一時的行為表現；would be（對未來或假設的推測）、was to be（命中注定、預定要）、would have been（與過去事實相反的假設）都與當下的實際表現無關。
be being + 形容詞。你腦中的 be 動詞只有一種讀法：接形容詞，講一個人是什麼樣的人，所以 was being truthful 看起來像是錯的。這一格考的是時態：進行式把時間框在某一個時間點，講的是那個當下的表現。Ken is kind.（Ken 人很好，講的是他這個人。）／When Ken pulled the door for me, he was being kind.（Ken 幫我拉住門的那一刻，人真好。）同一個形容詞，加了進行式就只講那一刻。要注意的是，能這樣用的只有描述舉止的形容詞——kind、rude、silly、careful、truthful；講感受或狀態的不行，英文不說 He is being happy 或 He is being hungry。
本題要判定的是受測者在測謊那段時間有沒有說實話，不是評斷這個人一輩子誠不誠實，所以用 was being truthful。文內把時間點寫得很清楚：Once the questions are finished（等問題問完之後）、the examiner analyzes the results（測謊人員分析結果），分析在後，回頭看的正是測謊進行中那一段。這個用法你國中見過：Don't be silly.（別傻了）／You're being silly.（你現在很幼稚）。
would be 是對未來或假設的推測；was to be 表「預定要」（The ceremony was to be held at noon. 典禮預定中午舉行）；would have been 是與過去事實相反的假設。三個都跟「測謊當下的實際表現」無關。
be being + 形容詞（那一刻的舉止）、tell the truth（說實話）、truthful（誠實的）、a lie detector（測謊器）、take a polygraph test（接受測謊）。
A polygraph machine, also known as a "lie detector," is a common part of criminal investigations. The instrument is used to measure 21 a person's body reacts to questions. The theory underlying it is that lying is stressful, and that this stress can be measured and recorded on a polygraph machine.
When a person takes a polygraph test, four to six wires, called sensors, are 22 to different parts of his body. The sensors pick up signals from the person's blood pressure, pulse, and perspiration. 23 the process of questioning, all the signals are recorded on a single strip of moving paper. Once the questions are finished, the examiner analyzes the results to determine if the person tested 24 truthful.
Well-trained examiners can usually detect lying with a high degree of 25 when they use a polygraph. However, because different people behave differently when lying, a polygraph test is by no means perfect.