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.
答案：(B) falling
【分詞作形容詞】falling on it 修飾 sunlight，表示「照射在它身上的陽光」，為主動意涵的現在分詞。falls 為限定動詞不可直接修飾名詞；fallen 為過去分詞，語意被動不合（sunlight 是主動落下）；to fall 不定詞不符合此處修飾名詞的用法。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
四個是同一個字的不同形態，會讓人立刻進入文法模式；又因為空格前面是 the sunlight，很多人覺得「陽光是被照下來的」而挑過去分詞 fallen。
先看整句的骨幹：Reflecting less than one percent of the sunlight ______ on it, TrES-2b is less reflective than ...。主要子句是 TrES-2b is less reflective ...，前面那一整串是分詞片語。空格前面是名詞 the sunlight、後面是介系詞片語 on it，可見空格填的是掛在 sunlight 後面的修飾語，不是限定動詞，falls 先出局。接著用修飾語後置的通則：分詞單獨一個時放在名詞前面（a barking dog），後面還帶著東西就整串移到名詞後面（the man standing there）。最後分主動被動：還原成 the sunlight that falls on it（照在它上面的陽光），是陽光自己落下來，主動關係，所以用現在分詞 falling。再補一個一勞永逸的判準：fall 是不及物動詞，本來就沒有被動式，所以只要看到 fall 的過去分詞被拿來表示被動，一律可以先刷掉。
falls 是限定動詞，這一句已經有主要動詞 is，不能再放一個。fallen 是過去分詞，但 fall 不及物、沒有被動用法，fallen 只能當形容詞用在「已經落下的」東西上（fallen leaves 落葉）；陽光是正在照過來，不是已經落地的東西。to fall 表目的或還沒發生的事，陽光不是「為了要照在它上面」才存在的。
the sunlight falling on ...（照在……上的陽光）、fallen leaves（落葉）、reflect（反射）、reflective（會反射的）、light years away（光年之遙）、a barking dog（吠叫的狗，分詞單獨一個放名詞前面）
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!