They grade and report on the quality of products, customer service, and environment of the stores 12 they are assigned.
答案：(D) to which
the stores to which they are assigned（他們被分配到的商店），關係代名詞 which 代替 stores，介系詞 to 來自 assigned to 這個字詞搭配；in that（因為）、with which（assigned是assigned to不是assigned with）、for that 都不合。
看到 the stores ___ they are assigned，你會想：這裡填關係代名詞 which 就好了，為什麼前面還要加 to？關鍵在於動詞片語本身自帶介系詞：assign sb to sth（把某人分派到某處）。把句子拆回兩句就清楚了——They grade and report on the quality... of the stores. 加上 They are assigned to the stores. 合併時 the stores 換成 which，原本跟在 assigned 後面的 to 也跟著搬到 which 前面，就成了 to which。
步驟是固定的：先看空格後面那個動詞需要什麼介系詞。 這裡是 be assigned to（被分派到），所以介系詞是 to。也可以用還原法檢查：把它改寫成口語常見的 the stores which they are assigned to，意思完全一樣，只是把介系詞留在句尾——這是同一個結構的兩種寫法。
with which 錯在介系詞：assign 搭配的是 to 不是 with。in that 是連接詞「因為」（＝because），後面要接完整句子表原因，但本句缺的是修飾 the stores 的關係子句。for that 更是不成立——在關係子句裡，介系詞後面只能接 which 或 whom，不能接 that，這一點沒有例外。
assign sb to sth（把某人分派到……）、be assigned to（被分派到）、report on sth（就……提出報告）、the way in which（……的方式）、the extent to which（……的程度）、the reason for which（＝why，……的原因）。
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.