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。
答對率 29%。
讀到 because adjustable alarms had been invented by the mid-19th century（可調式鬧鐘在十九世紀中葉就發明了），你自然會推論「那他們就是在用鬧鐘啊」，於是挑了 were mostly using。這個推論在本句範圍內完全合理——問題出在下一句。
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 把前面的推論整個推翻：鬧鐘雖然已經存在，一般工人卻買不起、也信不過，所以實際上並沒有普遍使用。因此正解必須表達「本來可以用，但實際上沒有用」。這是典型的跨句題，只讀本句一定選錯。
劍橋《English Grammar Today》在 could 條目指出：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）。本句 They could have used alarm clocks 就是「他們本來可以用鬧鐘」，隱含「但沒有」。
must be using 是對「現在」的強烈推測，時態不合；had hardly used 是「幾乎不曾用過」，與 because 帶出的理由自相矛盾；were mostly using 則直接被下一句推翻。
could have + p.p.（本來可以）、should have + p.p.（早該）、must have + p.p.（想必已經）、needn't have + p.p.（本來不必）。
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.