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。
答對率 29%。
句首 Unless 後面直接接過去分詞 forbidden，看起來像少了主詞和動詞，你就不敢選。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相同時，主詞連同 be 動詞可以一起省略。這正是國中學過的規則：While (he was) walking home, he saw an accident.。同類的還有 if necessary（若有必要）、when possible（可能的話）、unless otherwise stated（除非另有說明）。
主要子句說郵政署首度只在週一到週五送信；本句是這個決定的前提條件——「除非國會下令禁止」。前一句 This decision was announced on Wednesday without congressional approval.（這項決定未經國會批准）也在鋪陳同一件事：國會還沒表態。
When（當…時）與 Once（一旦）都表時間，語意不對；Lest 是「以免」，後面要接完整子句且用原形動詞。
unless（除非）、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）。
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.