Hair usually gets greasy when it has not been washed because it soaks up oil—hence the need for shampoo! 21 this oil-absorbing feature might not always be so great for our hygiene, it can be great for the environment.
答案：(B) While
語意讓步對比：「雖然這種吸油特性對我們的衛生可能不總是很好，但它對環境卻可能很棒」，While 在這裡表示「雖然」的讓步轉折；For（因為）、In case（以防）、As long as（只要）都不合。
while 你腦中第一個意思一定是「當……的時候」，所以看到句首的 While 會直覺去找「同時發生」的兩件事，結果怎麼讀都不通順。問題出在 while 當連接詞其實有兩個意思：一個是時間（在……的時候），另一個是表示對比或讓步（雖然、然而）（劍橋 English Grammar Today）。
看前後兩個子句的關係就好。前半：this oil-absorbing feature might not always be so great for our hygiene（這種吸油的特性對我們的衛生不見得是好事）；後半：it can be great for the environment（但它對環境可能很棒）。一個負面、一個正面，是明顯的對比，不是同時發生。驗算方式最簡單：把 While 換成 Although（雖然）讀一次，意思完全通，那就對了。表示對比的 while 也可以換成 whereas（然而）。
For（因為）表原因，但吸油特性對衛生不好，並不是它對環境好的原因。In case（以防萬一）要接「為了預防某事發生」。As long as（只要）是條件。這三個代進去，前後兩句的正負對比全部會消失。這個「因果還是對比」的判斷在 108 年第 25 格也考過（Due to 與 In contrast to 的選擇），做法一樣：先看前後兩句是同向還是反向，再挑連接詞。
while（雖然、然而，表對比；也可表「當……的時候」）、whereas（然而，表對比）、although（雖然）、even though（即使、雖然）、in case（以防萬一）、as long as（只要）、soak up（吸收）、be great for（對……很有好處）、hygiene（衛生）。
Hair usually gets greasy when it has not been washed because it soaks up oil—hence the need for shampoo! 21 this oil-absorbing feature might not always be so great for our hygiene, it can be great for the environment.
Matter of Trust, a nonprofit organization, has an innovative solution for removing the large-scale oil spill in the Gulf of Mexico in 2010, using the ultimate renewable 22 : human hair, of which there is an unlimited supply. Since its founding in 1998, Matter of Trust has collected donations of human hair and animal fur to 23 the thousands of oil spills that happen each year. The hair and fur are made into mats and brooms and sent to 24 waters to absorb the oil.
Across the United States each day, 300,000 pounds of hair and fur are cut. Matter of Trust is helping organize the collection of this 25 hair and fur through thousands of salons, pet groomers, and ranchers. Individuals can also speak to local hair stylists and pet groomers about sending in leftover hair and fur.