Green tea is also helpful 18 infection and damaged immune function.
答案：(A) for
【句型】helpful for 字詞搭配「對……有幫助」；from／at／inside 都不合。
helpful 你最熟的搭配是 helpful to me、helpful to you，後面接的都是人。這裡接的卻是 infection（感染）這種「狀況」，四個介系詞看起來好像都能塞。更麻煩的是，「對感染有幫助」中文聽起來怪怪的，反而讓人不敢選 for。
先把 for 這個用法對回你國中就會的句子：This medicine is good for a headache.（這個藥治頭痛。）for 後面接的是「要對付的問題」，不是「受益的人」。helpful for infection 就是同一個模子：對感染這個問題有效。詞典把這組介系詞分得很清楚：接人時用 to（helpful to parents，對家長有幫助）；接事情、問題或目的時用 for；接動作時用 in ＋ V-ing（helpful in developing communication skills，有助於培養溝通能力）（牛津高階英漢雙解詞典 helpful 詞條）。本句空格後面是 infection and damaged immune function，兩個都是要對付的狀況，所以用 for。
from：英文沒有 helpful from 這個搭配；from 表來源或起點，與「有幫助」接不上。at：good at（擅長）才是常見的組合，helpful at 不成立。inside：表「在……內部」的位置關係，填進去會變成「在感染裡面有幫助」，語意不成立。
be helpful for（對……有幫助、對……有效）、be helpful to somebody（對某人有幫助）、be helpful in V-ing（在……方面有幫助）、be good for（對……有益）、be good at（擅長……）、immune function（免疫功能）。
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.