The instrument is used to measure 21 a person’s body reacts to questions.
答案：(C) how
語意「用來測量一個人的身體對問題『如何』反應」，how（如何）符合；what／when／why 都不合這個描述反應方式的語境。
看到 measure（測量）就想到「測量什麼」，直覺填 what — 這是本題最大的陷阱，答對率只有 50%，等於一半的人栽在這裡。
選 wh- 詞最可靠的方法是「數成分」：先把空格後面的部分單獨拿出來，看它缺不缺東西。a person's body reacts to questions — 主詞 a person's body 有了，動詞 reacts 有了，to questions 這個介系詞片語把 react 補滿了，這是一個成分完整的句子。既然一樣都不缺，就不能用 what；what 進到子句裡一定要當主詞或受詞，會在句中留下一個空位。反向驗算最能看出差別：如果要用 what，句子得寫成 measure what a person's body reacts to（測量一個人的身體對什麼有反應）— 注意結尾的 to 後面空著，那個空位就是留給 what 去填的。原句的 to questions 已經填滿了，what 沒有位置可站。剩下 when、how、why 都是副詞性質，加進去不占句子成分，只能比語意：react 是「反應」，這台儀器量的是身體反應的「方式」（快慢、強弱），所以用 how。同一份考卷第 19 題也考 wh- 詞，但那題的著力點是「後面接名詞片語所以用 what」，這題的著力點是「子句成分完整所以不能用 what」— 同一年連考兩題，判斷方式不同，放在一起看效果最好。
when（何時）— 全文沒有在談時間點，測謊機也不是用來測「什麼時候反應」。why（為什麼）— 儀器記錄的是血壓、脈搏、排汗這些生理訊號，測不出動機。what — 如上所述，後面的子句成分完整，沒有空位留給它。
measure how…（測量……的方式）、react to（對……作出反應）、a reaction to（對……的反應）、be used to + 原形動詞（被用來……）、be used to + V-ing（習慣於……）、respond to（回應、對……有反應）、pick up signals（接收訊號）。
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.