Many thousand-year-old coins often sell for no more than a few dollars because there are a lot of them around, 23 a 1913 Liberty Head Nickel may sell for over one million US dollars because there are only five in existence.
答案：(B) while
語意對比：「許多千年古幣只值幾塊錢……而1913年自由女神鎳幣卻可能賣到超過一百萬美元」，while 在這裡表示對比「然而/而」；since（自從/因為）、whether（是否）、if（如果）都不合。
while 這個字，你的印象大概只有「當…的時候」。把「當…的時候」代進去，句子會變成「當 1913 年自由女神鎳幣可能賣超過一百萬美元時」，時間關係完全說不通，於是你回頭去挑 since 或 if。
while 當連接詞有三個常見用法，要靠上下文分辨（劍橋 English Grammar Today）：一是時間「當…的時候」，二是讓步「雖然」，三是本題的對比「而、然而」，用來把兩件相反或差距很大的事並排比較。本句兩邊的差距極大：一邊是千年古幣只賣幾塊錢（因為存世量大），一邊是 1913 年的鎳幣可以賣超過一百萬美元（因為只剩五枚）。金額從 a few dollars 到 over one million US dollars，數量從 a lot of them 到 only five，兩組數字擺明是拿來對照的。這正是對比用法的 while。判斷方法很簡單：看到 while，先問「兩邊是同時發生，還是拿來比較」。兩邊各有各的原因、各有各的結果，而且方向相反，就是對比。
since（自從、既然）：當「既然」用時表原因，但本句兩邊各自已經有 because 帶出自己的原因，再加一個表原因的連接詞會讓邏輯打結。whether（是否、不論是否）：引導名詞子句或讓步子句，本句不是在問是非，也不是「不論…」。if（如果）：表條件。1913 年鎳幣賣到百萬是既成事實，不是假設。
no more than（僅僅、不過）、in existence（現存的）、sell for（以…價格售出）、once in a while（偶爾）、a while ago（不久前）、worth a fortune（價值連城）。
For coin collectors who invest money in coins, the value of a coin is determined by various factors. First, scarcity is a major determinant. 21 a coin is, the more it is worth. Note, however, that rarity has little to do with the 22 of a coin. Many thousand-year-old coins often sell for no more than a few dollars because there are a lot of them around, 23 a 1913 Liberty Head Nickel may sell for over one million US dollars because there are only five in existence.
Furthermore, the demand for a particular coin will also 24 influence coin values. Some coins may command higher prices because they are more popular with collectors. For example, a 1798 dime is much rarer than a 1916 dime, but the 25 sells for significantly more, simply because many more people collect early 20th century dimes than dimes from the 1700s.