17 forbidden to do so by the Congress, the agency for the first time will deliver mail only Monday through Friday.
答案：(B) Unless
Unless forbidden to do so by the Congress 是省略了主詞與 be 動詞的讓步子句（＝Unless it is forbidden…），意為「除非國會禁止」；When、Once、Lest 語意都不合。
句首的連接詞後面直接接過去分詞 forbidden，看起來少了主詞和動詞，你就不敢下手。完整寫法是 Unless it is forbidden to do so by the Congress——從屬子句的主詞與主要子句一致時，主詞連同 be 動詞可以一起省略。
先把省略的部分補回來，句子的關係才看得清楚：Unless it is forbidden to do so by the Congress, the agency will deliver mail only Monday through Friday.補回來之後，前半是條件、後半是決定，兩者的關係是「除非發生這件事，否則就這麼做」。這種省略你國中就用過：While (he was) walking home, he saw an accident.（他走路回家時看到一場車禍。）本題只是把時間連接詞換成條件連接詞，把現在分詞換成過去分詞。同一個形狀還有一整組固定說法：if necessary（若有必要）、when possible（可能的話）、unless otherwise stated（除非另有說明）、if any（如果有的話）。文意也對得上：前一句說這項決定未經國會批准，代表國會尚未表態；本句正是在說「只要國會沒下令禁止，就照這樣執行」。驗算法：把 unless 換成 if…not 唸一遍——if it is not forbidden by the Congress（如果國會沒有禁止），意思不變，條件關係成立。
When 是「當…的時候」，會讀成「當國會禁止時，郵政署就只送週一到週五」——被禁止之後反而照做，前後矛盾。Once 是「一旦…就」，同樣把禁止當成觸發條件，矛盾之處與 When 相同。Lest 是「以免、免得」，兩個問題：它後面要接完整子句，不能像本句這樣省略主詞與 be 動詞；而且它的子句用原形動詞（lest + S + (should) + 原形 V），forbidden 這個形態也對不上。
unless（除非）＝ if…not、unless otherwise stated（除非另有說明）、if necessary（若有必要）、when possible（可能的話）、lest + S + (should) + V（以免）、congressional approval（國會批准）。
The U.S. Postal Service has been struggling financially for some time. It plans to stop delivering mail on Saturdays, 16 Aug. 1 this year. This decision was announced on Wednesday without congressional approval. 17 forbidden to do so by the Congress, the agency for the first time will deliver mail only Monday through Friday. It is expected that this 18 will save about $2 billion a year.
In recent years, the postal service has suffered tens of billions of dollars in losses 19 the increasing popularity of the Internet and e-commerce.
The postal service plans to continue Saturday delivery of packages, which remain a profitable and growing part of the delivery business. Post offices would remain open on Saturdays 20 customers can drop off mail or packages, buy postage stamps, or access their post office boxes. But hours would likely be reduced at thousands of smaller locations.