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。
答對率 38%。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）。
scientists tried to overcome the force of gravity（科學家設法克服重力）——overcome the force of gravity 與 shoot free of Earth's pull 是同一件事的兩種說法。後面的 in order to land humans on the moon 也確認方向是往外脫離地球。
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.