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 .
答案：(A) at all
if they are able to do so at all 中的 at all 用於條件句尾加強語氣，意為「究竟是否真能如此」；at hand（在手邊）、at first（起初）、at ease（自在）語意都不合。
at all 你大概只在否定句裡看過（not at all 一點也不），很少看到它單獨掛在條件句的句尾。這裡的 at all 是加強語氣，意思是「究竟、真的」。
先看空格落在哪裡。它在 if 子句的最後面——if they are able to do so ___，前面 do so 已經把動作講完了，空格補的是對這件事的態度，不是新的資訊。再看整句的語氣走向：In the absence of enzymes, reactants may take hundreds of years to convert into a usable product（沒有酶的話，反應物可能要花上幾百年才能轉換成可用的產物）——已經是很極端的說法，句尾的 if 子句是再加一層，語氣只會更強，不會轉去講別的事。at all 正是做這件事的：它把「要花幾百年」再推進成「而且還不確定究竟辦不辦得到」。驗算法：把 at all 換成中文的「究竟」唸一遍——如果它們究竟真辦得到的話，語氣接得上，方向就對。這種 if…at all 的用法在英文裡自成一組，都是「就算真有，也很有限」的口吻：I rarely, if ever, go there.（我幾乎不去那裡，如果真去過的話。）／There is little, if any, evidence.（就算有證據，也少得可憐。）
三個都是 at + 名詞的片語，形狀一樣，意思各自不同。at hand 是「在手邊、近在眼前」，講的是位置或時間上的接近，接在 do so 後面會變成「在手邊辦得到」，說不通。at first 是「起初」，表示時間上的第一階段，通常後面要接一個轉折（at first…, but later…），本句沒有這樣的對照。at ease 是「自在、不拘束」，形容人的狀態（feel at ease、put sb at ease），主詞是反應物，不是會感到自在的對象。
if…at all（如果真的…的話）、not at all（一點也不）、if any／if ever（如果真有／真做過）、at hand（在手邊；即將到來）、at ease（自在）、in the absence of（缺乏）、convert into（轉換成）。
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.