The local number, 17 , must always be dialed in its entirety.
答案：(D) on the other hand
前面講區域碼有時不用撥，這句轉折「另一方面」，本地號碼一定要完整撥打，on the other hand 符合這個對比；in fact（事實上）、to illustrate（舉例說明）、at the same time（同時）都不太符合這種對比關係。
四個選項都是常見的轉承語，代進去唸起來都不會卡，於是只好憑語感選。答對率只有 40%，最多人失手在 at the same time——因為前後兩句講的確實是同一通電話的兩個部分，看起來很像在講同時發生的事。
先把前後兩句在講什麼攤開來，再問「它們之間是哪一種關係」。前一句：同一區的話，區域碼通常不必撥。這一句：本地號碼一定得完整撥出來。一個「可以不撥」，一個「必須全撥」，兩條方向相反的規定擺在一起比較，這是對比，on the other hand 正是用來標示對比的另一面。on the other hand 可以單獨使用，不一定要先出現 on the one hand；只要前文已經給了可以拿來對照的那一面，就能直接用（劍橋 English Grammar Today）。本題前一句就是那一面。順帶注意它插在主詞 The local number 與動詞 must 之間，用兩個逗號夾住。轉承語出現在句子中間是很常見的位置，不要因為它沒放句首就把它排除掉。
in fact（事實上）：用來強化或修正前面說過的話，方向與前一句相同；本句方向相反。to illustrate（舉例說明）：後面要接前一句的具體例子，本句不是例子，是另一條規定。at the same time（同時、話說回來）：主要意思是「同時發生」，兩件事得在時間上並存。它雖然也有輕微的轉折用法，但強度遠不及 on the other hand，而本句是兩條規定的對比，不是時間上的並存。
on the one hand（一方面）、on the other hand（另一方面）、in fact（事實上）、as a matter of fact（事實上）、to illustrate（舉例說明）、at the same time（同時、話說回來）。
An area code is a section of a telephone number which generally represents the geographical area that the phone receiving the call is based in. It is the two or three digits just before the local number. If the number 16 is in the same area as the number making the call, an area code usually doesn't need to be dialed. The local number, 17 , must always be dialed in its entirety.
The area code was introduced in the United States in 1947. It was created 18 the format of XYX, with X being any number between 2-9 and Y being either 1 or 0. Cities and areas with higher populations would have a smaller first and third digit, and 1 as the center digit. New York, being the largest city in the United States, was 19 the 212 area code, followed by Los Angeles at 213.
In countries other than the United States and Canada, the area code generally determines the 20 of a call. Calls within an area code and often a small group of neighboring area codes are normally charged at a lower rate than outside the area code.