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。
答對率 22%。
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，接著空格，再接一個分詞片語 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就在空格。
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 ___，這一格顯然是全句唯一的主要動詞。redistributing 是現在分詞、being redistributed 是被動分詞，都不能單獨當主要動詞，先刪。
前一句 When on Earth, we have more fluids in our lower body… 用現在簡單式，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。
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。
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）。
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.