They may also pretend to be 14 customers, arguing with a salesperson without a good reason.
答案：(A) difficult
difficult 形容人時是「難相處的、難搞的」，a difficult customer 即「奧客」，與後面的分詞構句 arguing with a salesperson without a good reason 完全吻合；potential（潛在的）、constant（持續不斷的）、anonymous（匿名的）都不合。
difficult 你只記得「困難的」，用來形容事情（a difficult question）；形容「人」時它會轉彎，變成「難相處的、難搞的」——a difficult customer 就是「奧客」，a difficult child 是「難帶的孩子」。
答案寫在逗號後面。空格後緊接 arguing with a salesperson without a good reason（無緣無故就跟店員起爭執），這個分詞構句正是在說明前面那種顧客長什麼樣。看到空格後接逗號加分詞或同位語，先讀那一段，答案常常就藏在裡面。再確認一次方向：本句說神祕客有時會「假裝成」某一種顧客，而假裝的目的是測試店家的應對，所以那個形容詞必須描述一種會給店員出難題的顧客——difficult 正是這個意思。驗算法：把 difficult 換成「難搞的」唸一遍，接回後面的分詞構句，前後說的是同一件事，方向就對了。同類會轉彎的形容詞一起記：easy（容易的／隨和的，an easy person to work with）、hard（困難的／嚴厲的，be hard on sb）、tough（艱難的／強悍的）、mean（吝嗇的、刻薄的）。看到熟悉的形容詞去修飾人，先想它有沒有第二個意思。一句記完：Luckily, Adam was not a difficult person; when I admitted losing his camera, he only asked whether it could still be found.（幸好 Adam 不是難相處的人；我坦承弄丟了他的相機之後，他只問還找不找得到。）〔情境取材自 97 學測作文題：借來的貴重物品遺失〕
potential 是「潛在的」，a potential customer 指還沒消費、可能會來的人，但神祕客是進到店裡執行任務的，而且潛在顧客不會無故與店員爭執。constant 是「持續不斷的」，形容頻率或狀態（constant noise），用來修飾顧客會變成「不斷出現的顧客」，那是 a regular customer（常客）的意思，與後面的爭執接不上。anonymous 是「匿名的」，指不具名，神祕客確實不表明身分，但這一句講的是他假裝成哪一種顧客、做出什麼行為，不是講他具不具名；填進去後面那段爭執就沒有著落。
a difficult customer（難搞的顧客）、a difficult child（難帶的孩子）、a regular customer（常客）、a potential customer（潛在顧客）、mystery shopper（神祕客）、pretend to V（假裝做…）。
Mystery shoppers are paid consumers who are hired to shop in stores and collect data. They are sent to a wide variety of service-based businesses, including stores, restaurants, and banks—potentially anywhere customers are served. Sometimes referred to as secret shoppers, mystery shoppers 11 regular customers to purchase items, eat in restaurants, make inquiries, or return items.
They grade and report on the quality of products, customer service, and environment of the stores 12 they are assigned. When evaluating a business, mystery shoppers have to follow certain standard procedures to avoid 13 . They are often given a checklist that provides directions on what to observe or look out for to ensure consistency.
Sometimes these shoppers collect data about their "normal" observations, such as cleanliness of the store or timeliness of the service. They may also pretend to be 14 customers, arguing with a salesperson without a good reason.
With the data they collect, mystery shoppers can identify areas for 15 and thereby help enhance the quality of products and services. In this way, these secret shoppers may help a business gain a competitive edge.