It is held in unknown, guarded warehouses 23 only a few people can see it.
答案：(B) where
where 引導形容詞子句修飾地方先行詞 warehouses，表示「在那些倉庫裡，只有少數人能看到它」。which 需要子句中扮演主詞/受詞角色，此處子句已是完整句子不缺成分，故不合；until（直到）、unless（除非）都不符合修飾地點的語法功能。
which 與 where 中文都能說成「那裡」，你會憑語感挑一個。
鑑別度高達 61。關係詞的題目有一招最可靠：把先行詞代回子句裡，看它在子句裡以什麼形式出現，就用對應的那一個關係詞。這一題的先行詞是 unknown, guarded warehouses（不為人知、有人看守的倉庫），子句是 only a few people can see it。把倉庫代回去，句子要寫成 only a few people can see it in the warehouses——倉庫在子句裡是「在倉庫裡」這個地方副詞，既不是主詞也不是受詞，前面還得補一個介系詞。代回去變成地方副詞的，就用關係副詞 where。填進去讀成 It is held in unknown, guarded warehouses where only a few people can see it。這一招之所以可靠，是因為它連 whose 都涵蓋得住：the boy whose father is a doctor，把先行詞代回子句是 the boy’s father is a doctor，出現的形式是所有格 the boy’s，所以用 whose。代回去長什麼樣就用哪一個：當主詞用 who 或 which、當受詞用 whom 或 which、變成所有格用 whose、變成地方副詞用 where、變成時間副詞用 when。
which 是關係代名詞，代回去要能直接當主詞或受詞；但這一句代回去是 see it in the warehouses，倉庫前面少不了那個 in，所以 which 接不上（要用也得寫成 in which）。until（直到）與 unless（除非）是連接詞，填進去語意也不通——「直到只有少數人看得到」「除非只有少數人看得到」都不成句意。
where（關係副詞，先行詞代回子句後變成地方副詞）、whose（關係代名詞的所有格，代回子句後變成所有格）、in which 等於 where（在其中）、the reason why（……的原因）、be held in（被存放在）、a few（少數幾個）。
The world's largest collection of Khmer sculpture resides at Angkor, the former royal capital of Cambodia. The 7,000 pieces—full statues, heads and carved inscriptions—are breathtaking not only for their individual beauty, but also for 21 they represent: a road map to the origins of much of Southeast Asian culture.
But the collection is not in a museum, and it is not open 22 visitors. It is held in unknown, guarded warehouses 23 only a few people can see it. These people assembled it by removing figures from the ruins of Cambodia's world-famous temples and replacing them with hollow, concrete 24. For the most part, these are hard to distinguish from the originals.
Clever art thieves? No, these "art thieves" work for the Angkor Conservancy. 25 very large pieces too heavy to move, virtually all surviving Angkorian statues, wood carvings and artifacts have been replaced with copies.