The benefits include an easier visibility of the time, and an alarm that gets increasingly louder 24 turned off.
答案：(C) until
語意「鬧鐘會越來越大聲，直到被關掉為止」，until（直到）符合；when（當……時候）、as（當……同時）、since（自從）都不符合這種「持續到……為止」的語意。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
四個選項都是講時間的字，中文都能翻出「當……時、隨著……、直到……、自從……」，一個一個代進去唸都不算太怪。而且空格後面只有 turned off 兩個字、沒有主詞，你會覺得結構怪怪的，更難決定。
先把那個「怪怪的」結構還原：空格後面是 (it is) turned off，主詞 it 與 be 動詞 is 被省掉了，只留下過去分詞。這種省略你剛見過——91 年第 28 題的 as described in this story 用的是同一招，連接詞後面把主詞與 be 動詞一起省略（掛鉤 While (he was) walking home）。還原之後整句是 an alarm that gets increasingly louder until it is turned off。接著看語意：gets increasingly louder（聲音越來越大）描述的是一個持續進行的變化，這種變化需要一個終點來收尾——鬧鈴不會無止境地大下去，它一直大到有人把它關掉為止。四個選項裡只有 until 提供「一路持續到某個時間點為止」這個終點。
when（當……時）：只標出一個時間點，不含「一路持續到那個點」的意思，會變成「被關掉的時候聲音越來越大」，前後顛倒。as（隨著、當……時）：表示兩件事同步進行，同樣沒有終點的意思；而且「被關掉」是一瞬間的動作，不能與持續變大的過程同步。since（自從）：方向相反，它指的是起點不是終點，鬧鈴變大聲也不是從被關掉之後才開始。
until（直到……為止）、not … until（直到……才）、turn off（關掉）、turn down（把音量調小）、get louder（變大聲）、increasingly（越來越……地）。
At times, it seems like there are not many things that we can give a student who has everything. But 16 that Christmas is just a few weeks away, 17 is a gift idea. Those who have an inclination for crazy technological advances aren't hard 18. One simple way is a few new items that might 19 life more enjoyable. They don't look too revolutionary, but you'd be surprised 20 they are capable of. The following is one of the items.
21 a student who is never on time, there is the new Projection Alarm Clock. Many ordinary clocks are hard to read early in the morning, deceiving students of the correct time, 22 causing them to be late for school. Through a projector, this attractive radio-alarm-clock is able to display the time onto a wall, ceiling, or anywhere 23. The benefits include an easier visibility of the time, and an alarm that gets increasingly louder 24 turned off. It won't magically transport a student to school on time every day, but it just might make 25 a little easier.