Their 19 is in the processing.
答案：(C) difference
語意「它們的差異在於加工方式」，difference（差異）符合，呼應後文說明兩者處理過程不同；weight（重量）、purpose（目的）、structure（結構）都不合。
這一格多數人一眼就挑了 difference，靠的是語感。不過值得把判斷的依據想清楚，因為同樣的問法換一篇文章、換一組選項，語感就未必靠得住。
看空格前後的句子怎麼接。前一句 Green tea and black tea come from the same plant.（綠茶和紅茶來自同一種植物）已經把「相同」講完了；把兩樣東西擺在一起說「相同」之後，接下來自然要交代「那不同在哪」。空格所在的 Their ______ is in the processing 就是這個轉折點，填 difference 才接得上。再往後讀一句 Green tea is dried but not fermented（綠茶只乾燥不發酵），正是在說明兩者加工方式的差別，前後完全咬合。「先說相同、再說相異」是說明文最常見的推進方式，看到 the same plant 就要準備接 difference。
weight（重量）：全文沒有比較兩種茶的重量。purpose（目的）：processing 是製作過程，不是目的，而且茶葉本身不會有「目的」放在製程裡。structure（結構）：如果講的是葉片或分子結構，後面該接的是形狀或組成的描述，但接下來講的是乾燥與發酵這些「做法」，對不上。
the difference between A and B（A 與 B 之間的差別）、tell the difference（分辨出差別）、make a difference（造成影響、有所不同）、come from the same plant（來自同一種植物）、a lighter flavor（較清淡的風味）、be in the processing（在於加工過程）。
What is so special about green tea? The Chinese and Indians 16 it for at least 4,000 years to treat everything from headache to depression. Researchers at recently concluded that a compound in green tea 17 the growth of cancer cells. Green tea is also helpful 18 infection and damaged immune function. The secret power of green tea is its richness in a powerful anti-oxidant.
Green tea and black tea come from the same plant. Their 19 is in the processing. Green tea is dried but not fermented, and this shorter processing gives it a lighter flavor than black tea. It also helps retain the tea's beneficial chemicals. That is 20 green tea is so good for health.
The only reported negative effect of drinking green tea is a possible allergic reaction and insomnia due to the caffeine it contains.