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.
答案：(B) free of
shoot free of Earth's pull 意為「掙脫地球引力」，free of 是「擺脫、不受…束縛」；back to（回到）、long before（早在…之前）、straight on（一直向前）語意都不合。
這題很兩極：讀懂的人幾乎都對，沒讀懂的就全錯。
free 你大概只當它是形容詞「自由的、免費的」，沒把 free of 當成「擺脫、不受…束縛」記住：free of charge（免費）、free of pain（不再疼痛）、break free of tradition（掙脫傳統）。
空格前是動詞 shoot（衝出、射出），空格後是 Earth's pull（地球的引力），可見這一格要填的是兩者之間的方向關係：是朝向引力，還是脫離引力。作答線索在同一句前半：scientists tried to overcome the force of gravity（科學家設法克服重力），後半 in order to land humans on the moon（以便將人類送上月球）再次確認方向是向外離開地球。overcome the force of gravity 與 shoot ___ Earth's pull 為同一件事的兩種說法，兩相對照即可確定應填 free of。凡是考介系詞或副詞質詞的空格，判斷步驟相同：先在同句或鄰句尋找語意重複的另一種說法，方向即可確定。
back to 意為「回到」，方向相反，火箭並非衝回地球；long before 意為「早在…之前」，表達的是時間而非方向；straight on 意為「一直向前」，僅表示前進，不含脫離之意，且不與 shoot 構成此一說法。
free of／from（擺脫、免於）、break free of（掙脫）、free of charge（免費）、set sb free（釋放某人）。
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.