Plants make antioxidants to protect themselves from the sun's ultraviolet (UV) light, 18 may cause harmful elements to form within the plant cells.
答案：(A) which
which 引導非限定形容詞子句，先行詞是前面整個概念（紫外線），補充說明「紫外線可能導致植物細胞內產生有害物質」。逗點後的非限定子句不能用 that，what 不能有先行詞，such 不是關係代名詞，皆不合。
which 與 that 中文都能說成「這個、那個」，你大概覺得兩個都行，就挑一個讀起來順的。
先數句子成分：逗號後面是 ______ may cause harmful elements to form，動詞 may cause 缺一個主詞，所以空格要填的是一個能當主詞的關係代名詞，先行詞是逗號前的 the sun's ultraviolet (UV) light。接著看那個逗號：逗號隔開的是非限定關係子句，只是補充說明，不是用來限定先行詞是哪一個——而非限定關係子句不能用 that，只能用 which（劍橋 English Grammar Today 的 Relative clauses 條目）。還原一次驗算：..., and it may cause harmful elements to form within the plant cells。
that 在限定關係子句裡可以代替 which，但一碰到逗號就不行；what 自己帶著先行詞（等於 the thing that），這一句前面已經有 UV light 當先行詞，再放 what 就多了一個；such 不是關係代名詞，接不上。
逗號 ＋ which（非限定關係子句，只能用 which 不能用 that）、cause A to V（導致 A 做某事）、protect A from B（保護 A 免於 B）、antioxidants（抗氧化物質）、form（形成、產生）、ultraviolet light（紫外線）。
The fruits and vegetables we eat often come in distinctive colors. The rich colors, 16, are not there only to attract attention. They perform another important function for the plants. Research shows that the substances 17 these colors actually protect plants from chemical damage.
The colors come mainly from chemicals known as antioxidants. Plants make antioxidants to protect themselves from the sun's ultraviolet (UV) light, 18 may cause harmful elements to form within the plant cells.
When we eat colorful fruits and vegetables, the coloring chemicals protect us, too. Typically, an intensely colored plant has 19 of these protective chemicals than a paler one does. Research on how chemicals in blueberries affect brain function even suggests that these chemicals may help our own brains work more 20. In other words, eating richly colored fruits and vegetables makes us both healthier and smarter.