They 16 alarm clocks because adjustable alarms had been invented by the mid-19th century.
答案：(B) could have used
could have + 過去分詞表示「過去本來可以（但實際上沒有）」，呼應下一句 But the new device was still relatively expensive and unreliable；must be using（現在的推測）、had hardly used（幾乎不曾用過）、were mostly using（當時大多在使用）都與下文矛盾。
could have + 過去分詞。讀到 because adjustable alarms had been invented by the mid-19th century（因為可調式鬧鐘在十九世紀中葉就已經發明出來），你順著推：既然發明了，那他們就是在用鬧鐘。這個推論在本句範圍內完全合理，問題出在下一句。你熟的 could 是「能夠」，could have + 過去分詞講的是「當年本來可以，實際上沒有」，那個「沒有」不寫出來，藏在型式裡。
原句是 They (the mine and factory workers) could have used alarm clocks（礦坑與工廠的工人本來可以用鬧鐘）。下一句是 But the new device was still relatively expensive and unreliable.（但這種新裝置當時仍然昂貴又不可靠。）這個 But 把你的推論整個推翻：鬧鐘雖然存在，工人買不起也信不過，實際上並沒有在用。空格要的就是「本來可以用，但沒有用」。這是跨句題，只讀本句一定選錯——看到下一句由 But 或 However 起頭，先讀完再回頭填。文法依據見劍橋 English Grammar Today 的 could 條目：could have + 過去分詞用來談過去的可能性，例句 I could have been a lawyer.（我本來可以當律師的，言下之意是沒當成）。一句記完：My classmate could have joined the trip, but she had already spent her allowance.（我同學本來可以參加那趟旅行，但她的零用錢早就花完了。）〔情境取材自 98 指考作文題：不必擔心預算的一天〕
must be using 是對「現在」的推測，本段講的是十九世紀，時態就不合；had hardly used（幾乎不曾用過）與 because 帶出的理由自相矛盾；were mostly using（當時大多在使用）正是被下一句 But 推翻的那個說法。
could have + p.p.（本來可以…卻沒有）、should have + p.p.（早該…卻沒有）、must have + p.p.（想必已經）、needn't have + p.p.（本來不必…卻做了）。
The modern worker rolls out of bed, groans, and turns off an alarm clock before reluctantly getting up for the day’s work. But how did people get to work on time before alarm clocks were widely used? During the second Industrial Age, people toiled at unusual hours in mines or factories and often had to get up for work early in the morning. They 16 alarm clocks because adjustable alarms had been invented by the mid-19th century. But the new device was still relatively expensive and unreliable. British workers thus relied on a human alarm clock known as a “knocker-up.”
17 sticks or pea shooters, the human alarms would tour the streets, tap on windows, or blast them with dried peas, trying to wake paying customers in time for work. Whether they wielded rods or pea shooters, knockers-up became 18 throughout the United Kingdom. Every morning, these people, often older in age, were seen in big streets and small alleys, waking up their customers professionally. They usually would not leave people’s houses until they were sure their customers were awake.
While the practice continued in some parts of the country until the 1970s, it 19 as alarm clocks became more widespread and affordable. Today, beeping alarm clocks and smartphones that play morning music are surely simpler and more convenient. However, they cannot 20 the personal attention coming from the distinctive tap of a pea shooter.