This is why enzymes are crucial in the sustenance of life on earth. Enzymes, 19 , do not always function perfectly.
答案：(D) however
【轉折語】前句強調酵素對維持生命至關重要，後句卻指出酵素「並非總是完美運作」，語意轉折，故用 however（然而）。hereafter（此後）、instead（取而代之）、likewise（同樣地）皆不符合此處對比語境。
however 這個字，你的印象大概只有它出現在句首、後面接一個逗號（However, …）。本題它夾在主詞 Enzymes 與動詞 do not always function 之間，前後各一個逗號，位置一陌生就會覺得「這裡放不了 however」，轉而去挑看起來比較像插入語的字。
四個選項 hereafter、instead、likewise、however 都是副詞，文法上都插得進兩個逗號中間，可見這一格考的是前後兩句的邏輯方向。前一句 This is why enzymes are crucial in the sustenance of life on earth 說酵素對維繫地球生命至關重要，本句 Enzymes … do not always function perfectly 卻說它們並非總是完美運作，一正一反，要的是表相反的轉承語，四個選項裡只有 however 站在反方向。至於位置，therefore、however 這一類詞歸在「連接副詞」（linking adjunct）底下，可以出現在副詞能出現的任何位置（劍橋 English Grammar Today），插在主詞與動詞之間、前後用逗號隔開是很常見的寫法。鑑別度 44，分的是有沒有真的把前後兩句的方向擺在一起比對，而不是只憑哪個字看起來像插入語。
likewise（同樣地）要求前後同向，但前句在說酵素多重要、本句在說它會出錯，方向相反。instead（取而代之）要求「不是 A 而是 B」的替換關係，前一句並沒有被推翻掉。hereafter（此後、今後）是時間副詞，多見於法律與公文，本題前後兩句之間沒有時間先後可分。
not always（並非總是，部分否定）、function perfectly（運作良好）、be crucial in／to（對…至關重要）、This is why …（這就是為什麼…）、however 插在主詞與動詞之間（前後各用一個逗號隔開）、the sustenance of life（生命的維繫）
One factor that separates a living thing from an inanimate object is the organism's ability to carry out chemical reactions that are crucial for its survival. Imagine the infinite amount of reactions that a large organism such as human carries out every single day. 16 of these reactions are possible without enzymes.
Enzymes consist of various types of proteins that work to drive the chemical reactions 17 for certain types of nutrients to take effect. Enzymes can either launch a reaction or speed it up. In the absence of enzymes, reactants may take hundreds of years to convert into a usable product, if they are able to do so 18 . This is why enzymes are crucial in the sustenance of life on earth.
Enzymes, 19 , do not always function perfectly. In 1902 Sir Archibald Garrod was the first to attribute a 20 to an enzyme defect, which he later referred to as an "inborn error of metabolism." Today, newborns are routinely screened for certain enzyme defects such as PKU (phenylketonuria) and galactosemia, an error in the handling of the sugar galactose.