Post offices would remain open on Saturdays 20 customers can drop off mail or packages, buy postage stamps, or access their post office boxes.
答案：(A) so that
【連接詞】so that 引導目的子句，表示「郵局週六仍開放，以便讓顧客可以……」。as soon as（一…就）、in case（以防萬一）、ever since（自從）皆不符合此處的目的語境。
四個選項都由兩三個字組成、後面也都接子句，形態上刷不掉，你會一個一個代進去讀讀看。
這是邏輯勾環：要判斷的是前後兩件事之間的邏輯關係。前半是郵局週六仍照常開門，後半是民眾可以寄信寄包裹、買郵票、開自己的郵政信箱——後半正是前半的目的，開門就是為了讓民眾辦得成這些事。表目的的連接詞是 so that，而且它帶出的子句裡通常會有 can、could、may、will 這類助動詞，本句的 can 正好是它的標記。填進去讀成 remain open on Saturdays so that customers can drop off mail（週六仍開門，好讓民眾可以寄件）。
as soon as（一……就……）標示的是時間先後，但民眾辦事並不是郵局一開門就發生的某個單一事件；in case（以防萬一）後面要接的是想預防的狀況，民眾能寄信不是要預防的事，方向相反；ever since（自從……以來）要求後面接一個過去的起點並配現在完成式，本句講的是未來的安排。
so that ＋ S ＋ can／will（以便、好讓……）、in order that（以便）、in case（以防萬一）、as soon as（一……就……）、drop off（順道送去、放下）、remain open（維持營業、仍舊開著）。
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.