The fruits and vegetables we eat often come in distinctive colors. The rich colors, 16 , are not there only to attract attention.
答案：(C) however
however 作插入語表轉折，「這些鮮豔的顏色，然而，並不只是為了吸引目光」，帶出後文「顏色其實還有別的重要功能」的轉折語意。almost（幾乎）、rarely（很少）、relatively（相對地）都不符合這種轉折語氣。
however 這個字，你的印象大概只有「出現在句首」那一種擺法，看到它夾在主詞與動詞之間、前後各一個逗號，反而不敢選。
這是邏輯勾環，而且線索一定要跨句才看得到。鑑別度高達 60，分開的是認不認得 however 的這個位置。前一句是 The fruits and vegetables we eat often come in distinctive colors（我們吃的蔬果常有鮮明的顏色），讀到這裡的第一印象就是「顏色是為了好看」；空格這一句 The rich colors are not there only to attract attention（這些濃豔的顏色並不只是為了吸引目光）把那個印象翻過來，是轉折，所以填 however。位置的部分：however 屬於連接副詞，可以出現在副詞能出現的任何位置（劍橋 English Grammar Today），夾在主詞與動詞之間、前後各加一個逗號是常見的擺法。填進去讀成 The rich colors, however, are not there only to attract attention。
almost（幾乎）、rarely（很少）、relatively（相對地）三個都是副詞、都能插在這個位置，形態吻合，容易被順手選走，但都不表轉折：almost 是加強程度、rarely 是否定的頻率（讀成「很少不只是為了吸引目光」繞不通）、relatively 需要一個比較的基準。
however（然而，連接副詞，位置靈活）、nevertheless（儘管如此）、come in（有……的樣式或顏色）、attract attention（吸引目光）、perform a function（發揮功能）、relatively（相對地）。
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.