19 researchers are not completely sure, they believe it could be because its atmosphere is composed of light-absorbing chemicals like vaporized sodium or even some elements whose existence is not yet known.
答案：(C) While
【連接詞】While 在此表示「雖然／儘管」，引導讓步子句，表示「儘管研究人員並不完全確定，但他們相信……」。Once（一旦）、When（當…時）、If（如果）皆不符合此處表讓步對比的語境。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
四個都是從屬連接詞，後面都接得了子句，文法上刷不掉任何一個；When 與 While 的中文又都可以說成「當……的時候」，特別容易混。
看前後兩個子句的內容對不對得起來：______ researchers are not completely sure（研究人員並不完全確定），they believe it could be because ...（他們相信可能是因為……）。「不確定」與「仍然相信」是一正一反，這一格要的是讓步，不是時間、也不是條件。While 除了「當……的時候」，另一個常考的意思正是「雖然、儘管」（While I agree with you, I still have doubts.）。而這一篇給了最好的印證：下一句就是 Although they may never be able to tell exactly how ...，作者連著用兩個讓步句，一句 While、一句 Although。同一篇裡兩句功能相同的句子擺在一起時，後面那一句幾乎等於在替前面那一句的答案背書，這是很好用的驗算方式。
Once（一旦、一……就）表示某件事完成之後就會怎樣，但「不確定」不是一個會完成的事件。When 純講時間，讀成「當研究人員不確定的時候，他們相信……」語意很怪，兩件事並不是時間上的先後關係。If（如果）表條件，但研究人員不確定是文章直接陳述的事實，不是假設。
While ＋ 子句（雖然、儘管）、Although ＋ 子句（雖然）、Once ＋ 子句（一旦、一……就）、be composed of（由……組成）、be sure of（對……有把握）、light-absorbing（吸光的）
Astronomers have discovered the darkest alien planet known to exist in the galaxy.
The Jupiter-sized planet, known as TrES-2b, is located in another solar system some 750 million light years away from Earth. Reflecting less than one percent of the sunlight 16 on it, TrES-2b is less reflective than black acrylic paint, and is blacker than coal. The only light that comes from TrES-2b is a faint red glow, 17 the heated coils on an electric stove or a very dim light bulb. Researchers believe this results from its extremely hot atmosphere, which is as high as 1,800°F (980°C), 18 its closeness to its sun—its source of energy.
So how does this planet manage to absorb about 99% of the light it receives from its sun? 19 researchers are not completely sure, they believe it could be because its atmosphere is composed of light-absorbing chemicals like vaporized sodium or even some elements whose existence is not yet known. Although they may never be able to 20 exactly how the planet remains so stealthily hidden, scientists are sure of one thing—there are many more even darker planets out there just waiting to be discovered!