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。
答對率 31%。
at all 你大概只在否定句裡看過（not at all 一點也不），很少看到它出現在條件句的句尾。這裡的 at all 是加強語氣，意思是「究竟、真的」，常見於 if 子句之後：I rarely, if ever, go there.（我幾乎不去那裡，如果真去過的話）。
In the absence of enzymes, reactants may take hundreds of years to convert into a usable product, if they are able to do so at all.——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，「而且還不確定究竟辦不辦得到」。at all 就是在補上這層懷疑，語氣比只說「要花幾百年」更強。
at all（究竟／完全）、at hand（在手邊、即將到來）、at first（起初）、at ease（自在）、at once（立刻）、at best（充其量）。
if…at all（如果真的…的話）、not at all（一點也不）、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.