However, with the absence of gravity in the outer space, our body fluids 18 , shifting toward the upper body and the head.
答案：(A) redistribute
主要子句需要一個限定動詞，全文以現在簡單式敘述通則，故填原形 redistribute；redistributed 若當過去式則與全篇時態不合，redistributing 與 being redistributed 都是非限定形式，不能單獨當主要動詞。
句首一長串 However, with the absence of gravity in the outer space 讀完，主詞 our body fluids 才出現，接著就是空格，後面又接一個分詞片語 shifting toward the upper body——修飾成分把句子撐得很開，你就看不出主要動詞的位置正是空格。
先找主要動詞。把句首的介系詞片語與句尾的分詞片語都遮掉，剩下 our body fluids ___，這一格是全句唯一的主要動詞。redistributing 是現在分詞、being redistributed 是被動分詞，兩個都是非限定形式，當不了主要動詞，先刪掉。剩下 redistribute 與 redistributed，差別在時態。前一句 When on Earth, we have more fluids in our lower body…（在地球上，我們下半身的體液比較多）用的是現在簡單式，因為整段在說明科學通則；本句承接同一個通則，所以用原形。驗算法：把整段唸過一遍，看時態有沒有跳動——通則從頭到尾都用現在式，若這一格忽然變成過去式，就會讀成「曾經發生過一次」。順帶記一件事：redistribute 在這裡是不及物用法——體液自己重新分佈，後面不需要受詞。很多動詞（move、shift、spread）都兼具及物與不及物，看句子有沒有受詞就能分辨。一句記完：When my sister went to college, the housework in our family redistributed itself, and I ended up doing the dishes every night.（姊姊上大學之後，家裡的家事重新分配了一遍，結果每晚洗碗的變成我。）〔情境取材自 105 學測作文題：家事分工〕
redistributed 若讀成過去式，會把一個持續成立的科學通則說成過去發生過的單一事件，與全篇時態不合；若讀成過去分詞，則需要 be 動詞才能構成被動，本句沒有。redistributing 是現在分詞，只能當修飾語或接在 be 動詞後面構成進行式，單獨放在主詞後面會讓句子沒有動詞。being redistributed 同樣是非限定形式，而且多了被動的意思——體液是自己移動的，不是被誰重新分配的。
distribute（分配）／redistribute（重新分佈）、the absence of（缺乏）、shift toward（朝…移動）、be pulled down by（被…往下拉）、resemble（像、類似）。
Gravity has been at the top of the science agenda since the start of Mars missions. In the earlier days of space travel, scientists tried to overcome the force of gravity so that a rocket could shoot 16 Earth's pull in order to land humans on the moon. Today, they are more interested in how reduced gravity affects the astronauts' 17 condition. Our bodies have evolved to exist within Earth's gravity (1 g), not in the weightlessness of space (0 g) or the microgravity of Mars (0.3 g).
When on Earth, we have more fluids in our lower body because they are pulled down by Earth's gravity. However, with the absence of gravity in the outer space, our body fluids 18 , shifting toward the upper body and the head. As a result, the astronauts have swollen, puffy faces, very much resembling that of the round-headed Charlie Brown in the famous comic strip.
This "Charlie Brown effect" will be more 19 when the astronauts go on their Mars missions, which will take about three years to complete, much longer than missions to the moon. Moreover, the effect is often 20 space motion sickness, headaches, and nausea. Such a syndrome is considered the top health risk for the astronauts, and scientists are still trying to figure out how it may be prevented.