They can be 22 by their thin, light-colored skin.
答案：(C) identified
語意「可以透過薄而淺色的外皮辨識出它們」，identified（辨識）符合；grown（種植）、tasted（品嚐）、emphasized（強調）都不合。
四個都是過去分詞，形狀一模一樣。tasted 特別誘人，因為整篇都在講洋蔥的味道，而且下一句馬上出現 sweeter and milder tasting。
關鍵在 by 後面帶出來的東西：their thin, light-colored skin（薄而顏色淺的外皮）。外皮是用看的，不是用嚐的。by 在被動句裡帶出的是依據或方式——「憑著薄而淺色的外皮」，那是一個辨認的線索。identify 的意思是「辨識出、認出某物是什麼」，be identified by something 就是「可以憑某個特徵認出來」，整句是「可以從薄而顏色淺的外皮認出它們」。段落的走法也支持這個讀法：這一段先說新鮮洋蔥有哪些顏色，接著這一句講怎麼認出它，再往下才講味道（sweeter and milder tasting）。作者是照「外觀到口感」的順序寫的，這一句還停在外觀階段。
grown：種植，be grown by 會變成「由外皮種出來」，講不通。tasted：品嚐，外皮不是嚐味道的依據，而且味道要到下一句才登場。emphasized：強調，主詞是洋蔥，「洋蔥被外皮強調」語意不成立。
be identified by（憑…辨認出來）、identify A as B（辨識出 A 是 B）、light-colored（顏色淺的）、mild-tasting（口味溫和的）、bruise easily（容易碰傷）、an ideal choice for（…的理想選擇）。
Onions can be divided into two categories: fresh onions and storage onions. Fresh onions are available 21 yellow, red and white throughout their season, March through August. They can be 22 by their thin, light-colored skin. Because they have a higher water content, they are typically sweeter and milder tasting than storage onions. This higher water content also makes 23 easier for them to bruise. With its delicate taste, the fresh onion is an ideal choice for salads and other lightly-cooked dishes.
Storage onions, on the other hand, are available August through April. 24 fresh onions, they have multiple layers of thick, dark, papery skin. They also have an 25 flavor and a higher percentage of solids. For these reasons, storage onions are the best choice for spicy dishes that require longer cooking times or more flavor.