12 , fossil fuels are readily available and are currently plentiful.
答案：(B) Besides
前句提到化石燃料的第一項優點（燃燒即可產生大量能源），本句補充第二項優點（易取得、供應充足），屬「此外、還有」的遞進補充關係，故用 Besides；Likely（可能）不具句首轉承詞功能，In sum（總之）用於總結而非補充，On the contrary（相反地）語意相反，皆不符合。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
四個都是擺在句首的轉承說法，中文一個一個讀都像「而且、總之、相反地」，很容易憑語感挑。Likely 特別容易被讀成「很可能」而選它。
轉承語不能只看它自己的中文，要先確定前後兩句是同方向還是反方向，所以這一格一定要回頭讀前一句。前一句說燒了就能產生大量能源（優點一），這一句說 fossil fuels are readily available and are currently plentiful（容易取得、目前供應充足），還是優點。同方向再加一項，就是 Besides（此外、而且）。同一段的下一句 However 才是真正的反方向，兩個轉承語擺在一起看最清楚：Besides 往上加、However 往回轉。
Likely 是形容詞「可能的」，不是轉承語，不能單獨放在句首帶出一整句；要表達「很可能」得寫成 Most likely 或 It is likely that ...。In sum（總而言之）用在整段收尾做總結，但這一句是在補充新的一點，後面文章還很長。On the contrary 用來否定前面那句話（前面說 A，其實不是 A），這一句沒有推翻前一句，是在加碼。
besides（此外、而且）、in sum（總而言之）、on the contrary（正好相反）、be readily available（容易取得）、plentiful（充足的）、It is likely that ...（很可能……）
Modern societies have become increasingly vulnerable in their dependence on fossil fuels such as coal and petroleum. One 11 of using fossil fuels is that it can generate large amounts of energy by the simple process of burning. 12 , fossil fuels are readily available and are currently plentiful.
However, the burning of fossil fuels leads to the release of pollution into the atmosphere. Dependence on fossil fuels from unstable regions or countries creates energy security risks for dependent countries. Oil dependence 13 has led to monopolization, war, and socio-political instability.
Most importantly, fossil fuels are considered non-renewable resources, which will eventually decline in production and become 14 , with serious consequences to societies that remain highly dependent on them. Fossil fuels are actually slowly forming continuously, but we are using them up 15 approximately 100,000 times faster than they are formed.