Today, they are more interested in how reduced gravity affects the astronauts’ 17 condition.
答案：(A) physical
語意「低重力如何影響太空人的『身體』狀況」，physical（身體的）符合，呼應後文描述身體的具體生理變化（體液上移、臉部浮腫）；perceptual（知覺的）、mental（心理的）、external（外部的）都不合。
看到 condition（狀況），又想到太空人要執行長期任務，很容易聯想到「心理壓力」而選 mental。這是常見的先入為主：先想到自己覺得重要的面向，而不是文章實際在談的面向。
往後讀就知道整段談的都是身體：more fluids in our lower body（下半身體液較多）、our body fluids... shifting toward the upper body and the head（體液上移到上半身與頭部）、swollen, puffy faces（臉部腫脹浮腫）。從頭到尾沒有一句談情緒或心理，所以是 physical condition（身體狀況）。
mental（心理的）與 perceptual（知覺的）文章都沒有著墨；external（外部的）指外在環境，但 condition 前面的所有格是 the astronauts'，講的是太空人自身的狀況。
physical condition（身體狀況）、physical examination（健康檢查）、mental health（心理健康）、be in good condition（狀況良好）、on condition that（條件是……）、body fluids（體液）。
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.